Add pip-tools related-files to the project

We have to use a `requirements.in` just to proxy `setup.cfg`, since an
error occurs when we rely on the implicit dependency (ref:
jazzband/pip-tools#572).

This way abstracted requirements are expressed via `setup.cfg` while
concrete dependencies are expressed via `requirements.txt`.

Basic workflow:
- Add abstract dependencies to `setup.cfg`
- Proxy `setup.cfg` using `requirements.in`
- Write dev dependencies to `dev-requirements.in`
- Use `pip-compile --output-file (dev-)requirements.txt (dev-)requirements.in`
  to compile the concrete dependencies
- Add the `requirements.{in,txt}` files to source control for repeatable
  installations: https://caremad.io/posts/2013/07/setup-vs-requirement/
- Use `pip-sync dev-requirements.txt requirements.txt` to install the
  packages into the current virtualenv
- Run commands normally inside the current virtualenv (e.g. `tox`)
- Don't expose test requirements directly to pip-tools. Instead, just
  rely on tox/pytest-runner to install them inside the test venv.

dev-requirements.in

@@ -0,0 +1,2 @@
+tox
+ptpython

dev-requirements.txt

@@ -0,0 +1,18 @@
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile --output-file dev-requirements.txt dev-requirements.in
+#
+docopt==0.6.2             # via ptpython
+jedi==0.12.0              # via ptpython
+parso==0.2.1              # via jedi
+pluggy==0.6.0             # via tox
+prompt-toolkit==1.0.15    # via ptpython
+ptpython==0.41
+py==1.5.3                 # via tox
+pygments==2.2.0           # via ptpython
+six==1.11.0               # via prompt-toolkit, tox
+tox==3.0.0
+virtualenv==16.0.0        # via tox
+wcwidth==0.1.7            # via prompt-toolkit

requirements.in

@@ -0,0 +1 @@
+-e .

requirements.txt

@@ -1,7 +1,8 @@
-# Add your pinned requirements so that they can be easily installed with:
-# pip install -r requirements.txt
-# Remember to also add them in setup.cfg but unpinned.
-# Example:
-# numpy==1.13.3
-# scipy==1.0
-
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile --output-file requirements.txt requirements.in
+#
+-e file:///home/ab17624-l/workspace/dummy-pipenv-example-for-pyscaffold
+appdirs==1.4.3