DEPR: deprecate pytz support

[jbrockmendel]

We've already agreed on changing the defaults from pytz to stdlib/zoneinfo objects in 2.0. I'm now thinking we should go even further and deprecate support for pytz objects altogether. We wouldn't necessarily have to raise, but do something like

def disallow_pytz(tzinfo tz):
     if is_pytz_tzinfo(tz):
         return corresponding_zoneinfo(tz)
    return tz

The upshot of this is that we could remove a bunch of cython code that is pytz-specific. From there we would be able to de-duplicate conversion._localize_tso with tzconversion.tz_convert_from_utc_single.

[jbrockmendel]

@MarcoGorelli want to try to get something here for 2.2?

[MarcoGorelli]

I've had a new project come in at work so I don't think I have capacity to focus on this I'm afraid, I'm prioritising pdep6-related issues