Exclude .pyc in _vendor folder from sdist output

Fixes #1414

MANIFEST.in

@@ -2,7 +2,7 @@ recursive-include setuptools *.py *.exe *.xml
 recursive-include tests *.py
 recursive-include setuptools/tests *.html
 recursive-include docs *.py *.txt *.conf *.css *.css_t Makefile indexsidebar.html
-recursive-include setuptools/_vendor *
+recursive-include setuptools/_vendor *.py *.txt
 recursive-include pkg_resources *.py *.txt
 include *.py
 include *.rst

changelog.d/1533.misc.rst

+Restrict the `recursive-include setuptools/_vendor` to contain only .py and .txt files