build: Don't test C extension on CPython 2.7 under Tox (#406)

As the Changelog notes, release 1.0 will drop support for the native
extension on CPython 2.x. So there seems little benefit of testing it.

Author: moreati

tox.ini

@@ -1,5 +1,10 @@
 [tox]
-envlist = {py27,py35,py36,py37,py38}-{c,pure},{pypy,pypy3}-pure,py27-x86,py34-x86
+envlist =
+    py27-pure,
+    {py35,py36,py37,py38}-{c,pure},
+    {pypy,pypy3}-pure,
+    py27-x86,
+    py34-x86,
 
 [variants:pure]
 setenv=