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!!