Include pyproject.toml in sdist to avoid pytest warnings

Running tests from the current tarballs results in
"PytestUnknownMarkWarning: Unknown pytest.mark.requests".

The marker is defined in pyproject.toml which is missing.

Author: sbraz

MANIFEST.in

@@ -7,6 +7,7 @@ include CHANGES
 include CONTRIBUTORS.rst
 include LICENSE
 include README.rst
+include pyproject.toml
 include setup.cfg
 include setup.py