{Core} Fix _normalize_expires_on on Python 3.6

[jiasli]

Description

Fix #20215 (comment)

Python 3.6 doesn't recognize timezone %z as +00:00. It only recognizes +0000:

> docker run -it --rm python:3.6

>>> import datetime
>>> datetime.datetime.strptime('12/30/2021 07:53:52 +00:00', '%m/%d/%Y %H:%M:%S %z')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/_strptime.py", line 565, in _strptime_datetime
    tt, fraction = _strptime(data_string, format)
  File "/usr/local/lib/python3.6/_strptime.py", line 362, in _strptime
    (data_string, format))
ValueError: time data '12/30/2021 07:53:52 +00:00' does not match format '%m/%d/%Y %H:%M:%S %z'

>>> datetime.datetime.strptime('12/30/2021 07:53:52 +0000', '%m/%d/%Y %H:%M:%S %z')
datetime.datetime(2021, 12, 30, 7, 53, 52, tzinfo=datetime.timezone.utc)

which means az login --identity with managed identity in App Service container still doesn't work with DEB, RPM packages, etc.

https://docs.python.org/3/library/datetime.html#technical-detail

Changed in version 3.7: When the %z directive is provided to the strptime() method, the UTC offsets can have a colon as a separator between hours, minutes and seconds. For example, '+01:00:00' will be parsed as an offset of one hour. In addition, providing 'Z' is identical to '+00:00'.

There is indeed a test azure.cli.core.auth.tests.test_adal_authentication.TestUtil.test_normalize_expires_on to verify this:

azure-cli/src/azure-cli-core/azure/cli/core/auth/tests/test_adal_authentication.py

Lines 13 to 14 in af483f5

	assert _normalize_expires_on("11/05/2021 15:18:31 +00:00") == 1636125511
	assert _normalize_expires_on('1636125511') == 1636125511

However, the test is not executed by CI for Python 3.6 due to azdev issue

• Tests under subdirectories are not run azure-cli-dev-tools#318

[yonzhan]

Core