Steps to replicate the issue (include links if applicable):

ts1 = pywikibot.Timestamp(
    year=2022, month=12, day=21, hour=13,
    tzinfo=datetime.timezone(datetime.timedelta(hours=-5)))
t1 = pywikibot.WbTime.fromTimestamp(ts1, timezone=-300)
t1.toTimestamp() == ts1

What happens?: The equality is false.

What should have happened instead?: The equality is true

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.): For backwards compatibility, I'll introduce a parameter keep_timezone that adds the timezone in.