Refine the formatting of generated Python3 code

[pelson]

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This handles mostly whitespace improvement to be more aligned with PEP-8 by default. It doesn't yet fix a number of the newline issues that crop up.

Additionally, it improves the typing of a number of obvious methods. The typing remains incomplete however.

The changes to XPathLexer.py are fully auto-generated (with the exception of the checkVersion call).

In the future, I would work towards removing the from antlr4 import * and replacing it with named imports.

I have validated this against a mildly complex parser grammar in https://github.com/SciTools/cf-units/blob/main/cf_units/_udunits2_parser/udunits2Parser.g4.