dependecy specs: tolerate new PEP440 semantics too

dependecy specs: tolerate new PEP440 semantics too

The latest setuptools (version 8) changed the way dependency
specifications ("I can handle libfoo version 2 or 3, but not 4") are
interpreted. The new version follows PEP440, which is simpler but
somewhat less expressive. Tahoe's _auto_deps.py now uses dep-specs which
are correctly parsed by both old and new setuptools.

Fixes ticket:2354.

* Restrict the requirements in _auto_deps.py to work with either the old
  or PEP 440 semantics.
* Update check_requirement and tests to take account of changes for PEP
  440 compatibility.
* Fix an error message.
* Remove a superfluous TODO.