DEPR: parsing tzlocal depending on user's system timezone

[jbrockmendel]

# When run on a machine in located in US/Pacific timezone

>>> pd.Timestamp("2023-01-17 09:19 PDT")
Timestamp('2023-01-17 09:19:00-0800', tz='tzlocal()')

>>> pd.Timestamp("2023-01-17 09:19 EDT")
/usr/local/lib/python3.10/site-packages/dateutil/parser/_parser.py:1207: UnknownTimezoneWarning: tzname EDT identified but not understood.  Pass `tzinfos` argument in order to correctly return a timezone-aware datetime.  In a future version, this will raise an exception.
  warnings.warn("tzname {tzname} identified but not understood.  "
Timestamp('2023-01-17 09:19:00')

I'm fairly confident if @jreback ran the same examples while located in US/Eastern, he would get flipped results.

(Note also that I passed "PDT" for Pacific Daylight Time, even though we are on standard time. I'd get the same results with "PST/EST" instead of "PDT/EDT".)

I propose we deprecate recognizing this at all. i.e. cases that currently return tzlocal depending on your system timezone would raise in the future.

[mroeschke]

xref #18702