Fix PySlice_CheckFull on 32-bit architectures

[cjwatson]

When a stepping-forward slice has no upper bound, PySlice_Unpack sets stop to PY_SSIZE_T_MAX, not -1. They happen to be the same on some architectures, but that isn't portable.

Fixes: #1029

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.59%. Comparing base (029cfe8) to head (f9ba5af).

Files with missing lines	Patch %	Lines
native/python/pyjp_class.cpp	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1336      +/-   ##
==========================================
+ Coverage   86.53%   86.59%   +0.05%     
==========================================
  Files         113      113              
  Lines       10496    10496              
  Branches     4029     4029              
==========================================
+ Hits         9083     9089       +6     
+ Misses        814      807       -7     
- Partials      599      600       +1     

Files with missing lines	Coverage Δ
native/python/pyjp_class.cpp	85.36% <0.00%> (ø)

... and 2 files with indirect coverage changes

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