BUG/API: Raise when extension class passed to astype

[TomAugspurger]

Closes #17780

[TomAugspurger]

cc @jorisvandenbossche @jreback @toobaz

For the moment, I went with raising when an extension type class is passed instead of an instance. I'm happy to change the behavior here though if anyone disagrees. CategoricalDtype has an obvious "default" of CategoricalDtype(). DatetimeTZDtype doesn't really have a good default, so I decided to consistently raise an exception.

[codecov]

Codecov Report

Merging #17796 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #17796      +/-   ##
==========================================
- Coverage   91.24%   91.22%   -0.02%     
==========================================
  Files         163      163              
  Lines       49967    49971       +4     
==========================================
- Hits        45593    45588       -5     
- Misses       4374     4383       +9

Flag	Coverage Δ
#multiple	89.03% <100%> (ø)	⬆️
#single	40.24% <50%> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/core/internals.py	94.38% <100%> (ø)	⬆️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.74% <0%> (-0.1%)	⬇️

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