Bundle bs4.dammit revno 342 into thirdparty

Re: chfoo/wpull#170
Fix up thirdparty.dammit for Py3 and add copyright notice

re chfoo/wpull#170
string: Use our own bundled dammit module

re chfoo/wpull#170
Drop beautifulsoup4 requirement

Closes chfoo/wpull#170