Fix DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version.

[shcherbin]

This pull request includes updates to address a deprecation warning and increment the version number. The most important changes include updating the datetime.utcfromtimestamp method to datetime.fromtimestamp with timezone information and updating the version number.

Fixes for deprecation warning:

• pynamodb/attributes.py: Replaced datetime.utcfromtimestamp with datetime.fromtimestamp in the _normalize and deserialize methods to handle timezone information correctly. [1] [2]

Version update:

• pynamodb/__init__.py: Updated the version number from 6.0.1 to 6.0.2.

Documentation:

• docs/release_notes.rst: Added release notes for version 6.0.2, including the fix for the datetime.utcfromtimestamp deprecation warning.

[shcherbin]

@garrettheel, could you please find a moment to review this PR?
I'm using this library in one of my projects, and this Deprecation warning is driving me crazy :)

[ikonst]

Mind bumping the minor version and amending docs/release_notes.rst? Otherwise no new version will be released.

[shcherbin]

Thank you for looking into this, @ikonst. I made the changes you asked for. Please take another look.

[shcherbin]

I see the build fails.
Can it be related to actions/setup-python#962 ?
Based on the discussion there, it looks like 3.7 is no longer available on ubuntu runners. I can patch the actions pipeline in this PR.
Let me know what you think, @ikonst

[ikonst]

Thanks for fixing CI!

[shcherbin]

Thank you, @ikonst, for merging my PR :) When are you planning to roll out the next release?

[ikonst]

@shcherbin Done, thanks for the reminder. (I forgot to make the release. Just merging isn't enough.)