Remove hard-coded assert for number of users in leaderboard list.

A few pep8 nits:

    pep8 tests/test_leaderboard_page.py
    tests/test_leaderboard_page.py:28:53: E231 missing whitespace after ','
    tests/test_leaderboard_page.py:32:80: E501 line too long (90 > 79 characters)
    tests/test_leaderboard_page.py:35:80: E501 line too long (94 > 79 characters)