requirements: Force use of `html5lib` with a version that works.

This works for me and apparently causes the script to parse HTML better with
BeautifulSoup 4.

This should help with issues #131, #134, #137.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
test: fixtures: Add HTML from issue #93.
coursera: test: Enable tests for course from issue #93.
test: fixtures: Add HTML for datasci-001.
test: parsing: Add tests for parsing datasci-001.
test: Separate fixtures in directories to avoid clutter.
Add test for parsing startup-001 with bs4
PEP8: remove too many blank lines
Merge pull request #144 from jonasdt/parser_tests

Add tests from Jonas for startup-001.
Add test for parsing wealthofnations-001 with bs4