remove definitions of Python Source Code Encoding

I think PEP 0263 (http://legacy.python.org/dev/peps/pep-0263/) is
not used anywhere else and so it should be removed.

Change-Id: Ic2c2b284cc80e3db4380fda6f81d45f6d57f2d02
Merge "remove definitions of Python Source Code Encoding"
oslo.db.exceptions module documentation

Change-Id: Ie4a56a043215bc893e54f5d8b03bb28cb824548f