Remove default implementation for PyGCProtocol

[Alexander-N]

Implementations for these methods should always be provided. See #531

[codecov]

Codecov Report

Merging #532 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #532   +/-   ##
=======================================
  Coverage   87.55%   87.55%           
=======================================
  Files          65       65           
  Lines        3440     3440           
=======================================
  Hits         3012     3012           
  Misses        428      428

Impacted Files	Coverage Δ
src/class/gc.rs	94.11% <ø> (ø)	⬆️

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[codecov]

Codecov Report

Merging #532 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #532      +/-   ##
==========================================
- Coverage   87.68%   87.55%   -0.13%     
==========================================
  Files          65       65              
  Lines        3443     3440       -3     
==========================================
- Hits         3019     3012       -7     
- Misses        424      428       +4

Impacted Files	Coverage Δ
src/class/gc.rs	94.11% <ø> (-0.48%)	⬇️
src/instance.rs	96.66% <0%> (-1.34%)	⬇️
src/err.rs	61.11% <0%> (-1.24%)	⬇️

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[konstin]

Thank you!