Fix json-trait-rs usage of pyo3

[macisamuele]

ObjectProtocol has been removed by PyO3/pyo3#892

[codecov]

Codecov Report

Merging #44 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   96.40%   96.33%   -0.08%     
==========================================
  Files           9        9              
  Lines        1337     1338       +1     
==========================================
  Hits         1289     1289              
- Misses         48       49       +1     

Impacted Files	Coverage Δ
src/traits/_pyo3.rs	96.78% <ø> (ø)
src/rust_type_impl.rs	91.04% <0.00%> (-0.46%)	⬇️

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