make avro objects serializable & Pickle testing

[mr-c]

Fixes #659

[codecov]

Codecov Report

Merging #660 (eecd6a9) into main (075d185) will increase coverage by 0.05%.
The diff coverage is 81.48%.

❗ Current head eecd6a9 differs from pull request most recent head 96c37cc. Consider uploading reports for the commit 96c37cc to get more accurate results

@@            Coverage Diff             @@
##             main     #660      +/-   ##
==========================================
+ Coverage   83.12%   83.18%   +0.05%     
==========================================
  Files          22       22              
  Lines        4487     4490       +3     
  Branches     1239     1239              
==========================================
+ Hits         3730     3735       +5     
+ Misses        487      486       -1     
+ Partials      270      269       -1     

Impacted Files	Coverage Δ
schema_salad/codegen.py	94.59% <0.00%> (ø)
schema_salad/dotnet_codegen.py	95.77% <ø> (ø)
schema_salad/makedoc.py	84.75% <0.00%> (ø)
schema_salad/schema.py	78.38% <0.00%> (ø)
schema_salad/avro/schema.py	68.31% <90.90%> (+0.86%)	⬆️
schema_salad/cpp_codegen.py	78.81% <100.00%> (ø)
schema_salad/dlang_codegen.py	84.17% <100.00%> (ø)
schema_salad/fetcher.py	63.96% <100.00%> (ø)
schema_salad/java_codegen.py	91.10% <100.00%> (ø)
schema_salad/python_codegen.py	93.40% <100.00%> (ø)
... and 3 more

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