Remove SETUPTOOLS_SCM_PRETEND_VERSION during linting

It was needed because of check-manifest, but we no longer have a
MANIFEST file so it is not necessary

Author: nicoddemus

tox.ini

@@ -49,9 +49,6 @@ commands=
 skipsdist=True
 usedevelop=True
 basepython = python2.7
-# needed to keep check-manifest working
-setenv =
-    SETUPTOOLS_SCM_PRETEND_VERSION=2.0.1
 deps =
     flake8
     # pygments required by rst-lint