[WIP] Fix derive of PySequenceProtocol trait

[althonos]

As shown in #421, implementing PySequenceProtocol is not possible in v0.6.0 because the custom derive macro shuffles the argument types. This PR fixes the method types, and adds some tests to check a PySequenceProtocol implementor compiles as intended.

TODO:

• Fix argument types of __getitem__, __setitem__, __delitem__, __repeat__, __concat__
• Fix return types of __inplace_concat__ and __inplace_repeat__: it should be something like PyResult<&'mut Self> or PyResult<PyMutRef<Self>> since these methods do not have ownership over the Rust struct.

[codecov]

Codecov Report

Merging #423 into master will decrease coverage by 0.25%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master    #423      +/-   ##
=========================================
- Coverage   87.85%   87.6%   -0.26%     
=========================================
  Files          62      62              
  Lines        3285    3331      +46     
=========================================
+ Hits         2886    2918      +32     
- Misses        399     413      +14

Impacted Files	Coverage Δ
src/class/macros.rs	88.35% <100%> (ø)	⬆️
src/class/sequence.rs	80.8% <66.66%> (-15.63%)	⬇️
src/types/list.rs	97.14% <0%> (+0.12%)	⬆️
src/err.rs	62.42% <0%> (+1.81%)	⬆️

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[codecov]

Codecov Report

Merging #423 into master will decrease coverage by 0.25%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master    #423      +/-   ##
=========================================
- Coverage   87.85%   87.6%   -0.26%     
=========================================
  Files          62      62              
  Lines        3285    3331      +46     
=========================================
+ Hits         2886    2918      +32     
- Misses        399     413      +14

Impacted Files	Coverage Δ
src/class/macros.rs	88.35% <100%> (ø)	⬆️
src/class/sequence.rs	80.8% <66.66%> (-15.63%)	⬇️
src/exceptions.rs	64.7% <0%> (ø)	⬆️
src/type_object.rs	88.4% <0%> (ø)	⬆️
src/types/mod.rs	100% <0%> (ø)	⬆️
src/types/list.rs	97.14% <0%> (+0.12%)	⬆️
src/err.rs	62.42% <0%> (+1.81%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[konstin]

Thank you, looks good to me

[althonos]

Rebased on latest master. CI fails because of 9a9b519.