Remove bs4 deprecation warnings

[wileykestner]

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Recent versions BeautifulSoup4 have introduced some warnings for bs4 methods called by ofxparse.

This PR:

• updates the deprecated method usages to their recommended equivalents
• silences a warning about using bs4's HTML parser, as it's being used intentionally to preserve CDATA which gets dropped in lxml parsing mode
• removes test discovery via nose with the test discovery methods included in the Python standard library. This seems sufficient for the current needs ofofxparse and removes a dependency.

[wileykestner]

@jseutter Are you still maintaining this library? Would love to get this PR merged!