Rewrite timezone handling to support tzinfo zones, not fixed offsets

[teliseo]

• In addition to fixed offsets, tzinfo timezone names may now be used in the --tz and --move-zone options, and in the AuthorMap file.
• --tz now doesn’t just affect the log display, it changes the interpretation of the local timestamps in SCCS deltas. Without --tz, deltas are treated as in the system default timezone, or as specified by the TZ environment variable. In all cases, there is proper handling of daylight/summer time, which was previously lacking.
• --move-zone has changed from --move-offset. It is now a new timezone name or offset, not an hour offset from the previous zone.
• The AuthorMap parser is now much more robust, and can accept all forms of time zone specs.
• A timezone specified in the AuthorMap now takes precedence over --move-zone.
• Previously, specifying an AuthorMap file prevented fallback to system login name lookup, which it now does.
• Optimizations have been made to delta sorting.

This improved timezone handling doesn’t require any third-party modules, but ZoneInfo does require a fairly recent version of Python, 3.9, released in 2020. Everything should be golden on modern Posix-compatible systems, but Windows and some containers may lack a proper tzinfo backend, requiring additional modules.

For Windows specifically, depending on the local zone fallback will not properly handle daylight/summer time, but this can be mitigated by always specifying a tzinfo zone name with the --tz option. This could be improved by using the third-party tzlocal module, but I didn’t do this to avoid a third-party dependency.

[robohack]

This looks very interesting too.

I'll try it with my test case repository soon!

[teliseo]

I intend for you to merge my other PR (#1) first, which will cause a couple of merge conflicts with this one (around the option parsing). I’m not sure if I have the appropriate perms to fix them, or if you have to do this. I could always merge your updated master into this branch locally and then push, but I don't know if that's the cleanest github strategy.

After this, I think I’m done with major hacking on this project for a while, as it now meets my needs.