Fix Type Annotations in pandas/_version.py

[vaibhavhrt]

Part of #25882

Current errors are as follows:

pandas\_version.py:51: error: Need type annotation for 'LONG_VERSION_PY'
pandas\_version.py:52: error: Need type annotation for 'HANDLERS'

[vaibhavhrt]

@WillAyd I have decided to start from top in mypy.ini so here is the first one. please tag label, project and milestone for it because I don't have access to it.