BUG/API: Index creation with different tz coerces DatetimeIndex
Closes #11488.

Based on the type check added in #11588, the change affects to fewer functions than I originally thought.

Thanks, moved to `inferrence.pyx`. Also fixed and added tests for `NaT`.
 lgtm. comment above.
 Thanks, I've fixed the tz comparison logic based on your comment below.

> do an equality check on tz first then only check get_timezone if it's False
 merged via https://github.com/pydata/pandas/commit/b80b5c7443499905eda2e3295ed8973db12a9853

thanks!
