Handle the non-integer return of __index__

[HyeockJinKim]

Generate TypeError when index return non-integer value

Fixes#96

[codecov-io]

Codecov Report

Merging #97 into master will increase coverage by 0.26%.
The diff coverage is 85.71%.

@@ Coverage Diff @@## master #97 +/- ## ========================================== + Coverage 69.64% 69.91% +0.26%  ========================================== Files 60 60 Lines 10810 10817 +7 ========================================== + Hits 7529 7563 +34 + Misses 2750 2729 -21 + Partials 531 525 -6

Impacted Files	Coverage Δ
py/internal.go	44.13% <100%> (+2.34%)	⬆️
py/range.go	86.66% <84.61%> (-0.68%)	⬇️
py/slice.go	84% <0%> (+2%)	⬆️
py/list.go	50.36% <0%> (+3.64%)	⬆️
py/tuple.go	50.92% <0%> (+11.11%)	⬆️

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[HyeockJinKim]

@corona10
Can I get a review?

[ghost]

Does this also affect the list type?
CPython supports:

>>>l= [1,2] >>>classC: ... def__index__(self): ... return1 ... >>>l[C()] 2

If your change has an influence on this, you could add a test for list, too.

[HyeockJinKim]

@Tim-St
This fix affects strings, tuples, and lists.
So I added test code for each one. Would you like to review it again?

[ghost]

@HyeockJinKim Looks good, I think it should work now!

[ncw]

That looks great now - thank you :-)