OCSP stapling support

[chayim]

Pull Request check-list

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Description of change

Support for OCSP verification

[codecov-commenter]

Codecov Report

Merging #1820 (2794782) into master (4831034) will decrease coverage by 0.78%.
The diff coverage is 27.32%.

@@            Coverage Diff             @@
##           master    #1820      +/-   ##
==========================================
- Coverage   94.32%   93.53%   -0.79%     
==========================================
  Files          75       76       +1     
  Lines       15783    15962     +179     
==========================================
+ Hits        14887    14930      +43     
- Misses        896     1032     +136     

Impacted Files	Coverage Δ
redis/client.py	89.96% <ø> (ø)
redis/ocsp.py	0.00% <0.00%> (ø)
setup.py	0.00% <ø> (ø)
tests/test_ssl.py	54.12% <36.48%> (-37.54%)	⬇️
redis/connection.py	88.21% <58.33%> (-0.50%)	⬇️
redis/utils.py	88.88% <80.00%> (-1.44%)	⬇️
tests/conftest.py	91.62% <83.33%> (+0.49%)	⬆️
redis/exceptions.py	98.50% <100.00%> (+0.04%)	⬆️
tests/test_cluster.py	98.48% <0.00%> (-0.32%)	⬇️
tests/test_graph.py	89.88% <0.00%> (-0.04%)	⬇️
... and 1 more

---

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[yossigo]

@chayim Am I reading this wrong, or is this PR about OCSP validation support rather than OCSP stapling? I don't see anywhere the Certificate Status TLS extension here, which is what OCSP stapling is all about.

[chayim]

This support OCSP validation within the tunnel. The client doesn't currently handle direct stapling. That's a next step.