enable custom requests session

[schnoddelbotz]

Enable pysolr users to provide custom requests session (e.g. with retry enabled...)

[codecov]

Codecov Report

Merging #348 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #348   +/-   ##
=======================================
  Coverage   94.31%   94.31%           
=======================================
  Files           7        7           
  Lines        1338     1338           
=======================================
  Hits         1262     1262           
  Misses         76       76           

Impacted Files	Coverage Δ
pysolr.py	89.44% <100.00%> (ø)

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[acdha]

Handy, thanks!