build: Create tox environments using a known Cython version (#408)

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.

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

@@ -5,6 +5,7 @@ envlist =
     {pypy,pypy3}-pure,
     py27-x86,
     py34-x86,
+isolated_build = true
 
 [variants:pure]
 setenv=