Re-enable warnings as errors in unit nox session

[parthea]

In python 3.12, 3 warnings are emitted upstream from protobuf. The warnings are as follows:

<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: Type google._upb._message.MessageMapContainer uses PyType_Spec with a metaclass that has custom tp_new. This is deprecated and will no longer be allowed in Python 3.14.

<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: Type google._upb._message.ScalarMapContainer uses PyType_Spec with a metaclass that has custom tp_new. This is deprecated and will no longer be allowed in Python 3.14.

.nox/unit-3-12/lib/python3.12/site-packages/google/protobuf/internal/well_known_types.py:93
  /usr/local/google/home/partheniou/git/proto-plus-python/.nox/unit-3-12/lib/python3.12/site-packages/google/protobuf/internal/well_known_types.py:93: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    _EPOCH_DATETIME_NAIVE = datetime.datetime.utcfromtimestamp(0)

proto-plus-python/noxfile.py

Lines 51 to 53 in a9700cb

	session.run(
	"py.test",
	"-W=error",

In #400, W=error was removed and we're no longer treating warnings as errors in order to allow the presubmits to pass since it is an upstream issue.

See upstream issue protocolbuffers/protobuf#12186 . This issue tracks adding the -W=error flag to the unit nox session once the upstream issue is fixed.