Use a requests session per FinTSHTTPSConnection

[michaelgruenewald]

This creates a requests session per FinTSHTTPSConnection. This mostly results in requests reusing the HTTPS connection, leading to fewer roundtrips and lower latency overall. It would also allow cookie sharing, etc., but I don't think that matters at all for any FinTS stuff. Given that it's also an isolated session per FinTSHTTPSConnection, I don't see how this could have a negative impact.

Tested with my bank.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.85%. Comparing base (eb94706) to head (0b6a64b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #161   +/-   ##
=======================================
  Coverage   87.84%   87.85%           
=======================================
  Files          26       26           
  Lines        3341     3342    +1     
=======================================
+ Hits         2935     2936    +1     
  Misses        406      406           

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