Make timestamp-parsing utils platform-independent

[Tobotimus]

Resolves #1939.

In general, this should make timestamp-parsing mostly platform-independent, since the use of platform-dependent methods such as datetime.fromtimestamp() and datetime.astimezone() are no longer being used. Instead, timestamps are manually offset from the Unix epoch, and are also localised manually.

This PR comes with a new utility function, botocore.utils.parse_posix_timestamp, which is simply a more specific version of botocore.utils.parse_timestamp.

I also made a comment in the relevant tests to note that the tests seem to be written for machines where the timezone is UTC. Tests were failing for me before making any changes, until I ran the tests with the TZ=UTC env var.

[codecov-io]

Codecov Report

Merging #1970 into develop will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #1970      +/-   ##
===========================================
+ Coverage    92.73%   92.76%   +0.03%     
===========================================
  Files           53       53              
  Lines        10293    10301       +8     
===========================================
+ Hits          9545     9556      +11     
+ Misses         748      745       -3

Impacted Files	Coverage Δ
botocore/utils.py	98.34% <100%> (+0.01%)	⬆️
botocore/credentials.py	98.87% <0%> (+0.33%)	⬆️

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[github-actions]

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