sdist is missing `test/samples/line_numbers.md`

[mtelka]

The sdist at PyPI is missing the test/samples/line_numbers.md file and so the testing fails:

=================================== FAILURES ===================================
__________________________ TestLineNumbers.test_main ___________________________

self = <test_line_numbers.TestLineNumbers testMethod=test_main>

    def test_main(self):
        # see line_numbers.md for a description of how the test works.
        NUMBER_OF_LINE_NUMBERS_TO_BE_CHECKED = 13
>       with open("test/samples/line_numbers.md", "r") as fin:
E       FileNotFoundError: [Errno 2] No such file or directory: 'test/samples/line_numbers.md'

test/test_line_numbers.py:27: FileNotFoundError
=========================== short test summary info ============================
FAILED test/test_line_numbers.py::TestLineNumbers::test_main - FileNotFoundEr...
=================== 1 failed, 263 passed, 1 skipped in 0.81s ===================

[pbodnar]

@mtelka, thanks for the report. I've fixed the issue by the commit above, together with adding some more files to the built package.

Letting the issue open for a while, so that you or anyone else can provide feedback to the change.