Avoids clash w/ pure-Python 'timestamp' module on case-insensitive filesystems.

N.B:  the canonical way to import the ``TimeStamp`` class is now::

  from persistent.timestamp import TimeStamp

which will yield the class from the extension module (if available),
falling back to the pure-Python reference implementation.

Fix a couple of discrepancies in the Python persistence implementation
uncovered thereby.

The doctest snippets moved to docs/api/attributes.

- Moar OIDS-must-be-bytes.

- Moar SERIALS-must-be-bytes.

- Moar list()'ing of dict items.

- Bytes everywhere for OID / serials.

- Different obejct comparison semantics.

- Replace class advice 'implments' w/ 'implementer' decorator.

- Flatten 'items()' / 'keys()' iterators for testing.

- (Temporary) Disable C extension building under Py3k.

- Work around move of UserDict / UserList.

- Make test of 'cmp' conditional on PYTHON2.

- Make '__setslice__' / '__delslice__' conditional on PYTHON2.

- Work around rename of 'copy_reg' -> 'copyreg'.

- Fix deprecated class advice.


- Drop unneeded unicode literals.

Fixed a typo in PWRD.update found by coverage.