fix: adjust Timestamp.astimezone checks

[AstreaTSS]

Pull Request Type

• Feature addition
• Bugfix
• Documentation update
• Code refactor
• Tests improvement
• CI/CD pipeline enhancement
• Other: [Replace with a description]

Description

This PR adjusts the Timestamp.astimezone checks to both not error out on otherwise valid cases, and to directly use the timezone objects instead of replacing it with the local timezone if one is provided.

Changes

• See above. The "valid" cases were a timestamp that had a month that was less than 12, or a day that was less than 31, but was a year greater than 1969. How did we not encounter this earlier?

Related Issues

Test Scenarios

from datetime import Timestamp
Timestamp.fromtimestamp(9892325)

Python Compatibility

• I've ensured my code works on Python 3.10.x
• I've ensured my code works on Python 3.11.x

Checklist

• I've run the pre-commit code linter over all edited files
• I've tested my changes on supported Python versions
• I've added tests for my code, if applicable
• I've updated / added documentation, where applicable