Fix python-arango Client (de)serialization

[geenen124]

This PR allows a python-arango ArangoClient to be serialized and deserialized correctly using pickle. This is needed for usage in multi-process environments.

To solve this, two named functions replace the lambda functions for JSON serialization and deserialization in the client constructor, and the DefaultHTTPAdapter now correctly overrides its __attrs__ variable, including the latest timeout variables to support correct __getstate__ and __setstate__ calls.

[codecov-commenter]

Codecov Report

Merging #264 (bb2ccce) into main (8b09e07) will increase coverage by 0.42%.
Report is 10 commits behind head on main.
The diff coverage is 96.20%.

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@@            Coverage Diff             @@
##             main     #264      +/-   ##
==========================================
+ Coverage   98.58%   99.01%   +0.42%     
==========================================
  Files          26       26              
  Lines        3899     3958      +59     
==========================================
+ Hits         3844     3919      +75     
+ Misses         55       39      -16     

Files Changed	Coverage Δ
arango/client.py	98.41% <87.50%> (-1.59%)	⬇️
arango/collection.py	98.50% <90.00%> (+2.03%)	⬆️
arango/aql.py	95.04% <100.00%> (+0.04%)	⬆️
arango/cursor.py	100.00% <100.00%> (ø)
arango/database.py	99.86% <100.00%> (+<0.01%)	⬆️
arango/http.py	100.00% <100.00%> (ø)