Each TFS wrapper class implemented their own version of

parse_revision_spec(), and due to lack of tests, we had some

regressions in the move to Python 3, plus one wrapper that just never

returned correct results.
TFExeWrapper.parse_revision_spec() always returned None results, or

crashed. This was due to _convert_symbolic_revision() missing a

return statement, causing a None to always be returned. When fixed,

this was still returning a mix of integers and strings, where strings

are the expected type.
TEEWrapper.parse_revision_spec() worked, but produced a mix of

integers and strings.
TFHelperWrapper.parse_revision_spec() produced byte strings, which

weren't compatible with other calls on Python 3. It could also produce

error messages that were empty.
There was also some dead code in these.
These methods have all been updated to work correctly in all conditions.

Result types are all Unicode strings. Dead code has been removed. Errors

given defaults. Assertions added for type safety.
Unit tests have been added to test each of these based on simulated data.

Unit tests pass on Python 3.7-3.11.