Cache truststore SSLContext as load_verify_locations() is slow

[ichard26]

SSLContext can be reused across connections as per the Python docs:

SSLContext is designed to be shared and used by multiple connections. Thus, it is thread-safe as long as it is not reconfigured after being used by a connection.

In addition, requests has been using a global SSLContext and the sky hasn't fallen on them so I'm going to say this is pretty safe.

See also python/cpython#95031.

[ichard26]

I didn't include a news entry as the benefit is not significant, but when you combine all of the verify location/path loads in a typical run, they consume ~75 ms (~15ms/load) on my system. This eliminates one load from the self-check session, and sethmlarson/truststore#171 will eliminate one load from truststore's intialization. I may include a news entry for the optimization work in general once it's done.