Type checkers should give `datetime.datetime` for `np.datetime64 - pd.Timedelta`

[cmp0xff]

This is a bit of a mess, because I don't think that numpy can know whether it supports subtraction or not with a datetime:

>>> import numpy as np
>>> import datetime as dt
>>> ts_np = np.datetime64("2021-01-01")
>>> ts_dt = dt.datetime(2020,12,1)
>>> ts_np - ts_dt
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'datetime.date' and 'datetime.datetime'
>>> ts_dt = dt.datetime(2020,12,1,3,24)
>>> ts_np - ts_dt
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'datetime.date' and 'datetime.datetime'
>>> ts_np = np.datetime64("2021-01-01T03:24")
>>>
>>> ts_np - ts_dt
datetime.timedelta(days=31)
>>> ts_dt = dt.datetime(2021,12,1)
>>> ts_np - ts_dt
datetime.timedelta(days=-334, seconds=12240)

So if the numpy datetime64 does not specify hour/minute, then you can't do subtraction. But if it does have the hour/minute, you can. And their typing can't see the difference, which makes sense.

I suggest doing tests with a numpy type that has no hour/minute resolution (as in this PR) AND with hour/minute resolution.

Without the resolution, it will fail at runtime, but we can't detect it, so put it in if TYPE_CHECKING_INVALID_USAGE with the ignores that detect it (in this case, need a pyright ignore, but not a mypy one)

With the resolution, it will work at runtime, but we can't get the right type with either type checker (pyright will reject, mypy will fail in assert type), so put in appropriate ignores.

In both cases, include comments as to what is going on.

Note - I think pyright is wrong here in not having numpy type declarations figure out that np.datetime64.__sub__(pd.Timestamp) is valid from a type perspective, because pd.Timestamp is a subclass of datetime.datetime But the __sub__() in numpy will take precedence here because it has to allow __sub__(datetime.datetime)

Created a pyright issue: microsoft/pyright#11135

Originally posted by @Dr-Irv in #1492 (comment)