Limit reliance on manage.py and nose

[jaraco]

• Remove reliance on manage.py in tox execution.
• Switch from nosetests to pytest for the test runner.
• Convert nose imports to use the form expected by nose2pytest
• Ran nose2pytest
• Manually converted assert_raises and removed unused imports.
• Replace init with setup_method
• Remove deprecated U mode and then rely on default parameter.
• Fix ResourceWarnings in tests.

Closes #52 and enables testing on newer Pythons.

[jaraco]

Tests are failing on Windows, but that's outside the scope of this issue.