remove ancient "ignore pyproject" directive

this directive was added due to some miscellaneous issue
years ago but we now need this file to be present in source
dist.

Change-Id: Ia422c3f01accf9b96dd150b9f5596dc005fb7f0d

Author: zzzeek

MANIFEST.in

@@ -5,7 +5,4 @@ include README* LICENSE CHANGES* log_tests.ini tox.ini hash_port.py
 
 prune docs/build/output
 
-# omit pyproject so that pip installers
-# don't use poorly supported pep517 mode
-exclude pyproject.toml