Improve type annotations.

[pawelrubin]

This PR introduces some improvements of type annotations in the AvroModel class.

Most importantly, deserialize and fake methods, which now have correct and generic return types.

[codecov]

Codecov Report

Merging #109 (56e375f) into master (f17d4b2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #109   +/-   ##
=======================================
  Coverage   99.56%   99.57%           
=======================================
  Files           7        7           
  Lines         694      698    +4     
  Branches      110      110           
=======================================
+ Hits          691      695    +4     
  Misses          1        1           
  Partials        2        2           

Impacted Files	Coverage Δ
dataclasses_avroschema/types.py	100.00% <ø> (ø)
dataclasses_avroschema/fields.py	99.52% <100.00%> (ø)
dataclasses_avroschema/schema_generator.py	100.00% <100.00%> (ø)

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[marcosschroh]

Thanks @pawelrubin