Use mypy-friendly conditional import for zoneinfo

[KapJI]

Proposed change

After #50387 mypy fails locally for me with:

homeassistant/util/dt.py:12: error: Name 'zoneinfo' already defined (possibly by an import)  [no-redef]

That's probably because I use Python 3.9 locally. My idea is that CI didn't catch it, because zoneinfo is marked as Python 3.9+ and the first import was just ignored.

I think it's still nice to support Python 3.9 before CI is switched to it.

This is a known issue of mypy and using sys.version_info is suggested workaround: python/mypy#1153

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[KapJI]

@cdce8p if you want to improve pylint a little bit more.

Mypy understands blocks inside sys.version_info but pylint doesn't. But on the other side pylint doesn't complain about

try:
  import x
except ImportError
  import y

while mypy has this issue.

It'd be nice to support sys.version_info in pylint as well.

[cdce8p]

You could use the ignored-modules option, documentation or wait for pylint-dev/pylint#4468 😃

[KapJI]

@balloob as you authored migration to zoneinfo. What do you think?
This is the only place which doesn't pass mypy+python3.9 and this change doesn't look like a big deal.

[balloob]

Thanks for addressing this. I was already stuck with it hence I used the type ignore.