Change sequence comparison to be string like

Since Biopython 1.54 (May 2010, see https://github.com/biopython/biopython/commit/5297b605cf77b0fbcaa59b06d334829030acc550 and later https://github.com/biopython/biopython/commit/6b33337ce3fd1e96a1c0a0a2bd8fb791054b4953 for details) we have tried to give a warning when doing sequence comparison that a future Biopython release would move to string-like comparison, and suggesting explicit use of ``str(...)`` or ``id(...)`` for unambiguous comparison.

This pull request finally makes that change -- see discussion on https://redmine.open-bio.org/issues/2351

Note the fact the ``__cmp__`` is not used on Python 3, meaning our ``Seq`` and ``MutableSeq`` comparison there has default to plain object instance comparison - see https://github.com/biopython/biopython/commit/eaac3052f31938ed727222ab72dc7f20147d4832 - this change also replaces the obsolete ``__cmp__`` method.

Question: Should there be a warning in the comparison methods that this has changed?