pypi: correct examples and tests for "file_name" NOT "filename"

[pombredanne]

@sethmlarson reported this on matrix:

So it looks like the "file name" qualifier for PyPI PURLs isn't defined right. The definition says file_name and the examples are all filename, so... anyone know which it actually is? 🙈

To which I replied:

That's a bug that should have been clarified. It is correct in the main type definition, but the examples and tests are incorrect!
See

purl-spec/types/pypi-definition.json

Line 31 in 96d6f8c

"key": "file_name",

^ is correct

purl-spec/types/pypi-definition.json

Line 38 in 96d6f8c

"pkg:pypi/django@1.11.1?filename=Django-1.11.1.tar.gz",

... in incorrect in examples

and also in tests
https://github.com/package-url/purl-spec/blob/main/tests/types/pypi-test.json#L99

[pombredanne]

Note that this is somehow a regression from the PR from @MarkLodato

• pypi: fix filename -> file_name example #415