Add timeout option to Client

[jpolchlo]

Related Issue(s):
Closes #127

Description:
In order to set an explicit timeout for some requests, this PR adds a timeout parameter to Client.open. This is still a work in progress, but I've been able to add a functioning timeout parameter; the remaining work includes proper test coverage (at the moment, it's not clear how to catch the error raised by this feature) and an additional pass over the API to see other places where this feature would need to be added.

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[gadomski]

The timeout type signature is complicated enough that it's probably worth defining a type alias, a la https://github.com/python/typeshed/blob/47ce3cb8dbf564130701b2344c24ff248675f6a2/stubs/requests/requests/sessions.pyi#L106. It should be a "public" type alias, unlike the typeshed (i.e. Timeout, not _Timeout).

[codecov-commenter]

Codecov Report

Patch coverage: 81.81% and project coverage change: -0.14 ⚠️

Comparison is base (0a43803) 87.51% compared to head (df8f496) 87.38%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
- Coverage   87.51%   87.38%   -0.14%     
==========================================
  Files          12       12              
  Lines         857      864       +7     
==========================================
+ Hits          750      755       +5     
- Misses        107      109       +2     

Impacted Files	Coverage Δ
pystac_client/stac_api_io.py	91.20% <77.77%> (-1.18%)	⬇️
pystac_client/client.py	81.96% <100.00%> (ø)

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[gadomski]

LGTM, I added a drop-dead simple smoke test just to ensure things don't break when you provide a timeout. Thanks!

[gadomski]

Sorry, I pushed "approve" prematurely. One inline change, and also Client.open needs timeout added to its docstring.