GCS authentication and object updated timestamp parsing fix for Python 3.8-3.10

[twarberg]

About this change - What it does

• When reading credentials from file required scopes was missing and GCS would respond with 401 Invalid Credentials
• Pryor to Python 3.11 datetime.fromisoformat only supported a specific set of formats which does not include the format returned by GCS (https://docs.python.org/3/library/datetime.html#datetime.datetime.fromisoformat)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.41%. Comparing base (9358041) to head (c71c0b3).
⚠️ Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
rohmu/object_storage/google.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #195    +/-   ##
========================================
  Coverage   73.41%   73.41%            
========================================
  Files          34       34            
  Lines        4085     4228   +143     
========================================
+ Hits         2999     3104   +105     
- Misses       1086     1124    +38     

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