Parsing a timestamp-string without a timezone-offset using `FromJsonString` throws a confusing message

[caje731]

What version of protobuf and what language are you using? Version: v3.17.3. Language:Python 3.8

What operating system (Linux, Windows, ...) and version? macOS Big Sur Version 11.2

What runtime / compiler are you using (e.g., python version or gcc version) Python 3.8

What did you do? Steps to reproduce the behavior:

>>> from google.protobuf.timestamp_pb2 import Timestamp 
>>> date_string = "2021-06-15T09:45:49"   # timezone-naive
>>> pb_timestamp = Timestamp().FromJsonString(date_string)

What did you expect to see

ValueError: time data '2021-06-15T09:45:49' does not specify a timezone-offset

What did you see instead?

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/x/.local/share/virtualenvs/y-i29MmNV0/lib/python3.8/site-packages/google/protobuf/internal/well_known_types.py", line 167, in FromJsonString
    date_object = datetime.strptime(second_value, _TIMESTAMPFOMAT)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/_strptime.py", line 568, in _strptime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/_strptime.py", line 349, in _strptime
    raise ValueError("time data %r does not match format %r" %
ValueError: time data '2021-06' does not match format '%Y-%m-%dT%H:%M:%S'

The time-data passed in was 2021-06-15T09:45:49, and not just 2021-06 as erroneously reported in the stack-trace.

Anything else we should know about your project / environment

[K-Yo]

This does not work:

https://github.com/protocolbuffers/protobuf/blob/fb8862f781430f2ebf66dae8ab6ac455ee778f40/python/google/protobuf/internal/well_known_types.py#L148

maybe changing it to this would do the trick?

 timezone_offset = value[10:].rfind('-')

took me a while to track down this bug to end up on this issue

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