On each page the shortcuts '[' and ']' will take you to the previous and
next files respectively. On the index page they take you to the final
and first files respectively.

Test cases:

    $ pytest --cov-report html --cov=. tests.py

in a directory with just tests.py, then with one, two or three .py
files.

Tested on Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:99.0) Gecko/20100101 Firefox/99.0

Pressing 'u' while on a file will now take you back to index.html, like
on Gerrit.