Add RetryStacIO

[gadomski]

Related Issue(s):

• Closes New feature: StacIO that retries network requests #958

Description:

Though I initially thought I could do this without urllib3, as I was implementing I decided it was silly to just copy the functionality from urllib3, since it's so commonly on systems w/ PySTAC. So I added it as an extras_require. The urllib3 typing story is a bit of a mess, so I had to use a couple of # type: ignores. Testing retries is more trouble than its worth IMO, so I only included smoke tests for reading with the new class.

Background:

requests uses urllib3, so I consider urllib3 a pretty no-brainer library to optionally depend on -- it's everywhere.

Sidecar:

• Includes two small doc fixes I found along the way
• Casually adds pystac-vcr as a test dependency, with an eye towards Avoid network calls in tests suite #949
• I rearranged requirements-test.txt because its arbitrary layout was getting to me ... now it's just a sorted list

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[pjhartzell]

Looks good.