py: fix import on Windows

[aisk]

Currently gpython can't handle import on Windows because it's using path to handle file paths, which only support OSes which use / as path separator:

PS C:\Users\xxxxx\Source\gpython> go run . py\tests\string.py
Traceback (most recent call last):
  File "C:\\Users\\xxxxx\\Source\\gpython/py\\tests\\string.py", line 5, in <module>
FileNotFoundError: 'Failed to resolve "libtest"'
exit status 1

Using path/filepath can fix it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.97%. Comparing base (79bb925) to head (db36b42).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #241   +/-   ##
=======================================
  Coverage   74.96%   74.97%           
=======================================
  Files          79       79           
  Lines       19107    19125   +18     
=======================================
+ Hits        14324    14339   +15     
- Misses       4105     4106    +1     
- Partials      678      680    +2     

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[ncw]

Looks great - thanks :-)