Remove costly python requirements on Raspberry Pi (#7635)

It takes up to 1 hour to boostrap on Raspberry Pi, largely due to adding
"pandas" which is expensive to build and doesn't have a linux aarch64
wheel on pypi. Remove packages with no wheels from the requirements on
aarch64 to save time (pandas, gdbgui).

This switches the compiled requirements.txt to a "constraints.txt" which
is like a requirements file but only contains version constraints and
does not automatically install its contents. That way, we can specify
what version to use for packages on amd64 without forcing their
installation on the Pi. See pip help install for more details on this
feature.

This reduces initial Pi 4 Python bootstrap down to ~4m from ~50m.

.gn

@@ -25,4 +25,5 @@ script_executable = "python3"
 
 default_args = {
   pw_unit_test_AUTOMATIC_RUNNER = "$dir_pigweed/targets/host/run_test"
+  pw_build_PIP_CONSTRAINTS = ["//scripts/constraints.txt"]
 }

scripts/constraints.txt

@@ -0,0 +1,205 @@
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile --output-file=constraints.txt requirements.txt
+#
+anytree==2.8.0
+    # via -r requirements.txt
+appdirs==1.4.4
+    # via
+    #   -r requirements.txt
+    #   virtualenv
+appnope==0.1.2
+    # via -r requirements.txt
+backcall==0.2.0
+    # via ipython
+bidict==0.21.2
+    # via python-socketio
+bitstring==3.1.7
+    # via -r requirements.esp32.txt
+brotli==1.0.9
+    # via flask-compress
+certifi==2021.5.30
+    # via requests
+cffi==1.14.5
+    # via cryptography
+chardet==4.0.0
+    # via requests
+click==8.0.1
+    # via
+    #   -r requirements.esp32.txt
+    #   flask
+    #   pip-tools
+colorama==0.4.4
+    # via west
+coloredlogs==15.0.1
+    # via -r requirements.txt
+cryptography==3.4.7
+    # via -r requirements.esp32.txt
+cxxfilt==0.2.2
+    # via -r requirements.txt
+dbus-python==1.2.16 ; sys_platform == "linux"
+    # via -r requirements.txt
+decorator==5.0.9
+    # via ipython
+distlib==0.3.2
+    # via virtualenv
+docopt==0.6.2
+    # via pykwalify
+ecdsa==0.17.0
+    # via -r requirements.esp32.txt
+filelock==3.0.12
+    # via virtualenv
+flask-compress==1.10.0
+    # via gdbgui
+flask-socketio==2.9.6
+    # via gdbgui
+flask==0.12.5
+    # via
+    #   flask-compress
+    #   flask-socketio
+    #   gdbgui
+future==0.18.2
+    # via
+    #   -r requirements.esp32.txt
+    #   mobly
+gdbgui==0.13.2.0 ; platform_machine != "aarch64"
+    # via -r requirements.esp32.txt
+gevent==1.5.0
+    # via gdbgui
+greenlet==1.1.0
+    # via gevent
+humanfriendly==9.2
+    # via coloredlogs
+idna==2.10
+    # via requests
+intelhex==2.3.0
+    # via -r requirements.txt
+ipython-genutils==0.2.0
+    # via traitlets
+ipython==7.24.1
+    # via -r requirements.txt
+itsdangerous==2.0.1
+    # via flask
+jedi==0.18.0
+    # via ipython
+jinja2==3.0.1
+    # via flask
+lockfile==0.12.2
+    # via -r requirements.txt
+markupsafe==2.0.1
+    # via jinja2
+matplotlib-inline==0.1.2
+    # via ipython
+mobly==1.10.1
+    # via -r requirements.txt
+numpy==1.20.3
+    # via pandas
+packaging==20.9
+    # via west
+pandas==1.2.4 ; platform_machine != "aarch64"
+    # via -r requirements.txt
+parso==0.8.2
+    # via jedi
+pep517==0.10.0
+    # via pip-tools
+pexpect==4.8.0
+    # via ipython
+pgi==0.0.11.2 ; sys_platform == "linux"
+    # via -r requirements.txt
+pickleshare==0.7.5
+    # via ipython
+pip-tools==6.1.0
+    # via -r requirements.txt
+portpicker==1.4.0
+    # via
+    #   -r requirements.txt
+    #   mobly
+prompt-toolkit==3.0.18
+    # via ipython
+protobuf==3.17.3
+    # via -r requirements.txt
+psutil==5.8.0
+    # via
+    #   -r requirements.txt
+    #   mobly
+ptyprocess==0.7.0
+    # via pexpect
+pycparser==2.20
+    # via cffi
+pyelftools==0.27
+    # via -r requirements.esp32.txt
+pygdbmi==0.9.0.2
+    # via
+    #   -r requirements.esp32.txt
+    #   gdbgui
+pygments==2.9.0
+    # via
+    #   gdbgui
+    #   ipython
+pykwalify==1.8.0
+    # via west
+pyparsing==2.3.1
+    # via
+    #   -r requirements.esp32.txt
+    #   packaging
+pyserial==3.5
+    # via
+    #   -r requirements.esp32.txt
+    #   mobly
+python-dateutil==2.8.1
+    # via
+    #   pandas
+    #   pykwalify
+python-engineio==4.2.0
+    # via python-socketio
+python-socketio==5.3.0
+    # via flask-socketio
+pytz==2021.1
+    # via pandas
+pyyaml==5.4.1
+    # via
+    #   mobly
+    #   west
+reedsolo==1.5.4
+    # via -r requirements.esp32.txt
+requests==2.25.1
+    # via -r requirements.txt
+ruamel.yaml.clib==0.2.2
+    # via ruamel.yaml
+ruamel.yaml==0.17.9
+    # via pykwalify
+six==1.16.0
+    # via
+    #   anytree
+    #   ecdsa
+    #   protobuf
+    #   python-dateutil
+    #   virtualenv
+timeout-decorator==0.5.0
+    # via mobly
+toml==0.10.2
+    # via pep517
+traitlets==5.0.5
+    # via
+    #   ipython
+    #   matplotlib-inline
+urllib3==1.26.5
+    # via requests
+virtualenv==20.4.7
+    # via -r requirements.txt
+watchdog==2.1.2
+    # via -r requirements.txt
+wcwidth==0.2.5
+    # via prompt-toolkit
+werkzeug==0.16.1
+    # via flask
+west==0.11.0
+    # via -r requirements.txt
+wheel==0.36.2
+    # via -r requirements.txt
+
+# The following packages are considered to be unsafe in a requirements file:
+# pip
+# setuptools

scripts/requirements.esp32.txt

@@ -0,0 +1,12 @@
+setuptools>=21
+click>=5.0
+pyserial>=3.0
+future>=0.15.2
+cryptography>=2.1.4
+pyparsing>=2.0.3,<2.4.0
+pyelftools>=0.22
+gdbgui==0.13.2.0 ; platform_machine != 'aarch64'
+pygdbmi<=0.9.0.2
+reedsolo>=1.5.3,<=1.5.4
+bitstring>=3.1.6
+ecdsa>=0.16.0