Fix initialisation of function, staticmethod and classmethod __dict__

[ncw]

Add test files for function, staticmethod and classmethod

Fixes #56

[codecov-io]

Codecov Report

Merging #57 into master will increase coverage by 1.49%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   66.44%   67.94%   +1.49%     
==========================================
  Files          59       59              
  Lines       10379    10378       -1     
==========================================
+ Hits         6896     7051     +155     
+ Misses       2990     2828     -162     
- Partials      493      499       +6

Impacted Files	Coverage Δ
py/staticmethod.go	61.11% <100%> (+49.34%)	⬆️
py/classmethod.go	55% <100%> (+44.47%)	⬆️
py/function.go	96.09% <100%> (+70.13%)	⬆️
py/type.go	51.22% <0%> (+1.63%)	⬆️
py/tuple.go	31.13% <0%> (+6.6%)	⬆️
py/internal.go	40.86% <0%> (+7.21%)	⬆️
py/none.go	52.38% <0%> (+9.52%)	⬆️
py/dict.go	54.66% <0%> (+10.66%)	⬆️
... and 2 more

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[corona10]

LGTM!!