Add support for retry-after header

[gmertes]

Add support for the Retry-After header to robust. I placed it behind an optional flag respect_retry_header, so default behaviour is unchanged. It's a bit of a niche feature, but it could be useful when using robust directly in an api client.

Supports both seconds and http date format of the header.

Includes tests.

[FussyDuck]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 55.55556% with 16 lines in your changes missing coverage. Please review.

Please upload report for BASE (develop@a9be1e2). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
multiurl/http.py	0.00%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #12   +/-   ##
==========================================
  Coverage           ?   14.66%           
==========================================
  Files              ?       14           
  Lines              ?     1064           
  Branches           ?        0           
==========================================
  Hits               ?      156           
  Misses             ?      908           
  Partials           ?        0           

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