BUG: Correct Timestamp localization with tz near DST (#11481)

[mroeschke]

• closes Wrong hour of DST end for Europe TZ #11481, Bug: Timestamp removes timezone localization #15777
• tests added / passed
• passes git diff upstream/master --name-only -- '*.py' | flake8 --diff
• whatsnew entry

Seemed that the Timestamp constructor with a string incorrectly localized a naive time with tz_convert_single instead of tz_localize_to_utc which is similar to how DatetimeIndex localizes.

Now the Timestamp will raise an error if an ambiguous combination of string and timezone is passed. Should this be the default behavior?

I had to slightly change 2 existing tests. test_timestamp_to_datetime_tzoffset used a now ambiguous timestamp and test_setitem_with_tz_dst had an incorrect expected timestamp.

[codecov]

Codecov Report

Merging #15934 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15934      +/-   ##
==========================================
+ Coverage   90.99%   90.99%   +<.01%     
==========================================
  Files         145      145              
  Lines       49570    49570              
==========================================
+ Hits        45106    45107       +1     
+ Misses       4464     4463       -1

Flag	Coverage Δ
#multiple	88.76% <ø> (ø)	⬆️
#single	40.56% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/tseries/index.py	95.43% <0%> (+0.09%)	⬆️

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[jreback]

hmm there as to be an error in tz_convert_single. though it might simply be best to just replace that function to a call exactly like this, IOW (and see if anything else breaks)

cdef tz_convert_single(....)
   return tz_localize_to_utc(.....)[0]

[jreback]

can you give a 1 sentence on what the error was

[jreback]

can you scour the xref'ed issues for any more test cases and see if this closes any linked issues? (if you did already then ok).

[mroeschke]

Added some additional tests and described the error.

Addtionally, I tried dropping in tz_localize_to_utc to replace tz_convert_single and things were breaking. I think tz_convert_single is necessary in some places to convert from UTC to local timestamps (e.g. to show local time attributes instead of UTC time attributes)

[jreback]

thanks @mroeschke added some more tests. will merge on pass.