Add Slice function for range type

[HyeockJinKim]

Crate a new range object by calculating start,
stop, and step when slice is entered as argument
to the getitem function of the range

Fixes#77

[codecov-io]

Codecov Report

Merging #83 into master will increase coverage by 0.13%.
The diff coverage is 95.83%.

@@ Coverage Diff @@## master #83 +/- ##
==========================================
+ Coverage 68.84% 68.98% +0.13% 
==========================================
Files 59 59 Lines 10545 10588 +43 ==========================================
+ Hits 7260 7304 +44 + Misses 2775 2774 -1 
Partials 510 510

Impacted Files	Coverage Δ
py/range.go	87.09% <95.83%> (+6.84%)	⬆️
py/internal.go	41.34% <0%> (+0.48%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 261242c...6ee9326. Read the comment docs.

[corona10]

Please add unittests in here

https://github.com/go-python/gpython/blob/master/py/tests/range.py

[corona10]

Looks good to me
Do you have any suggestion? @ncw

[ghost]

CPython (v3.7.4) has an additional type check that raises an exception on wrong types like:
TypeError: slice indices must be integers or None or have an __index__ method.
That means types having __index__ method operate on this.
For example:

classC:
def__index__(self):
return1r=range(10)
assertlen(r[:C()]) ==1# len(r[:1.1]) -> Exception

[corona10]

@HyeockJinKim Please rebase the PR and reflect @Tim-St 's review.

[corona10]

Please follow the comment I left.

[HyeockJinKim]

@Tim-St@corona10
Can I get a review again?

[corona10]

LGTM

[ghost]

@HyeockJinKim
Your code for __index__ doesn't check the return type.

For example:

classC:
def__index__(self):
return"abc"r[:C()]

Should raise TypeError: __index__ returned non-int (type str).

Maybe it would be good to first call __index__ using py.Call and then check error and then the type of the returned result if no error.

Also the following should throw an Exception like in CPython:
r[:1.1] # TypeError: slice indices must be integers or None or have an __index__ method

But I think that is quite much work to implement, so we could also just handle this as a new issue.

The following code results are different in CPython (sorry, I didn't find this before):

r=range(10)
r[-2::-2] # r[-2::2] is ok r[-2::-1]
r[:-2:-1]
r[-2:-2:-2]