Draft: Enable parsing of extension-less files

[JosiahOne]

It's somewhat common for python scripts to be stored on disk as if they were executables (i.e. with no extension).

However, the current is_pysource function and properties explicitly check for a few known python extensions which causing pydeps to fail on such scripts (due to a assert target.is_pysource call in DummyModule.

This PR proposes adding a condition to assume any extension-less file is in fact a python script. I've added a test case to verify the change.

[codecov-commenter]

Codecov Report

Merging #154 (f159a1e) into master (72003a3) will decrease coverage by 0.08%. The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
- Coverage   73.65%   73.57%   -0.09%     
==========================================
  Files          17       17              
  Lines        1435     1438       +3     
  Branches      302      304       +2     
==========================================
+ Hits         1057     1058       +1     
  Misses        299      299              
- Partials       79       81       +2     

Impacted Files	Coverage Δ
pydeps/dummymodule.py	69.86% <25.00%> (-3.00%)	:arrow_down:
pydeps/target.py	95.38% <100.00%> (ø)
pydeps/depgraph.py	75.35% <0.00%> (+0.35%)	:arrow_up:

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[JosiahOne]

To try to address code coverage I consolidated what appeared (to me) to be redundant logic. target.py now uses the is_pysource() function from dummymodule.py.

[JosiahOne]

Oof, I see my test is incorrect. I'm checking that for an empty set where I don't want to.

I tried to look at fixing that, but there are other issues (almost certainly this was the reason the old assertions existed for checking whether files ended with .py to begin with). ModuleFinder.run_script() is used under-the-hood to resolve imports, but this method seems to only support files ending with .py. Or at the very least, pydeps interaction with this function is failing on files not ending with .py even though the file content is identical. I'll look into this some more.