Fix astype from tz-aware type to tz-aware type

[mroeschke]

Description

closes #16973

Also matches astype from tz-naive to tz-aware type like pandas

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[galipremsagar]

/merge

[davidwendt]

Should the title of the PR be Fix astype from tz-naive type to tz-aware type?
(Just for posterity and the changelog)

[mroeschke]

Should the title of the PR be Fix astype from tz-naive type to tz-aware type?

The larger bug is described correctly by the PR title (which is also related to the linked issue). This PR also fixes a minor bug that you're describing, Fix astype from tz-naive type to tz-aware type, where an exception should have been raised.