build: Create tox environments using a known Cython version

This change causes Tox to run the project's setup.py in a virtualenv
(default path is .tox/.package). The required version of Cython is
installed, rather than whatever version is installed system wide. I
found that this eliminated test suite failures under CPython 3.8, that
were due to me having a version of Cython that was too old to generate a
compatible _cmsgpack.cpp.

isolated_build was released in Tox 3.3.0, released 2018-09-11.

e.g.

± tox -r
.package create: /home/alex/src/msgpack-python/.tox/.package
.package installdeps: setuptools >= 35.0.2, Cython~=0.29.13, pytest
py27-pure recreate: /home/alex/src/msgpack-python/.tox/py27-pure
py27-pure installdeps: pytest
py27-pure inst:
/home/alex/src/msgpack-python/.tox/.tmp/package/1/msgpack-1.0.0rc1.tar.gz
...
py27-x86 create: /home/alex/src/msgpack-python/.tox/py27-x86
ERROR: InterpreterNotFound: python2.7-x86
py34-x86 create: /home/alex/src/msgpack-python/.tox/py34-x86
ERROR: InterpreterNotFound: python3.4-x86
_________________________________________ summary
_________________________________________
  py27-pure: commands succeeded
  py35-c: commands succeeded
  py35-pure: commands succeeded
  py36-c: commands succeeded
  py36-pure: commands succeeded
  py37-c: commands succeeded
  py37-pure: commands succeeded
  py38-c: commands succeeded
  py38-pure: commands succeeded
  pypy-pure: commands succeeded
  pypy3-pure: commands succeeded
ERROR:  py27-x86: InterpreterNotFound: python2.7-x86
ERROR:  py34-x86: InterpreterNotFound: python3.4-x86

Author: moreati

pyproject.toml

@@ -0,0 +1,8 @@
+[build-system]
+requires = [
+    # Also declared in requirements.txt, if updating here please also update
+    # there
+    "Cython~=0.29.13",
+    "setuptools >= 35.0.2",
+]
+build-backend = "setuptools.build_meta"

requirements.txt

@@ -1 +1,2 @@
+# Also declared in pyproject.toml, if updating here please also update there
 Cython~=0.29.13

tox.ini

@@ -1,5 +1,6 @@
 [tox]
 envlist = {py27,py35,py36,py37,py38}-{c,pure},{pypy,pypy3}-pure,py27-x86,py34-x86
+isolated_build = true
 
 [variants:pure]
 setenv=