TYP: fix almost all keyword-only reported by stubtest

[twoertwein]

Ran poe stubtest --nightly | grep keyword-only

Not sure about the cases with

Not actually positional-only, used to handle deprecations in 1.5.0

Currently fails because of python/mypy#14311 (edit: I duplicated the overloads to avoid the mypy bug for now)

[gandhis1]

Sometimes keyword-only arguments are used when they are not actually such because that is the only way to handle some literal overloads: python/mypy#5486

I'm surprised the tests pass after removing some of these.

[Dr-Irv]

Not entirely sure how we test all these kinds of changes, but I checked that what you did matches the docs.

[Dr-Irv]

Remaining two issues are to put back the args in Timestamp.__new__() and delete __init__() from Resampler and BaseGroupBy

[Dr-Irv]

thanks @twoertwein