Update datetime parser to support negative timestamps

[mlbiche]

Change Summary

Update datetime parser to support negative timestamps. Negative timestamps corresponds to dates before 1970/1/1.

Related issue number

Close #1600

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1605 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1605   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3796      3796           
  Branches       752       752           
=========================================
  Hits          3796      3796           

Impacted Files	Coverage Δ
pydantic/datetime_parse.py	100.00% <100.00%> (ø)

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[samuelcolvin]

LGTM.

I'll merge when I've been through other PRs.

[samuelcolvin]

thanks.