packaging: include tox.ini in sdist

tox testing works without full git repo, so no obvious harm in
aiding people running it from source distribution

based on setuptools itself doing it, and suggesting so in an example in
docs:
https://setuptools.pypa.io/en/latest/userguide/miscellaneous.html#controlling-files-in-the-distribution
pypa/setuptools@551eb7f

beware of .gitattributes / MANIFEST.in precedence when using setuptools-scm

MANIFEST.in

@@ -5,6 +5,7 @@ include README.rst
 include THANKS
 include requirements_dev.txt
 include requirements_test.txt
+include tox.ini
 recursive-include tests *
 recursive-include examples *
 recursive-include docs *