Include .flake8 in PyPI tarball

It is needed for running the tests.

Author: Robert Schütz

MANIFEST.in

@@ -2,4 +2,4 @@ recursive-include docs *.py *.rst
 recursive-include examples *.py
 recursive-include tests *.py *.pem
 recursive-include asyncpg *.pyx *.pxd *.pxi *.py *.c *.h
-include LICENSE README.rst Makefile performance.png
+include LICENSE README.rst Makefile performance.png .flake8