chore(retry-after): add support for Retry-After header

[sshishov]

What type of PR is this? (check all applicable)

• [+] Feature

Description

Added support for Retry-After header which has "special" handling inside urllib3

Related Issues

• Closes #768

PR checklist

Before submitting this pull request, I have done the following:

• [-] Read the contributing guidelines
• [-] Ran tox and pre-commit checks locally
• [-] Added my changes to the CHANGES file

Added/updated tests?

N/A

[markstory]

It would be good to have a test for this scenario.