Use unsigned constants for second arithmetic in C timestamp

Rename SCONV in timestamp.py too, and change _UNSIGNED_ into _UINT32_ in C to better express the expected range.

Use names and 'functions' for the various constants and packing/unpacking operations for better readability.

This might fix #86

I also find that it makes the data derivation/reconstruction more
clear by making the powers-of-two factors more obvious.

Make the C and Python TimeStamp round the same way

Fixes #41. Starting with a test that failed.

Don't touch the C code, just make the Python version round-trip
through the bytes values like the C code does. This moves the
truncation of the seconds value to microseconds to the timeTime accessor.

Include OID and jar in default repr of Persistent; let subclasses implement _p_repr for exception safety

 Remove unneeded _compat._b/_u/_native.

Also some minor DRY in test_picklecache.py

Fixes #81.

Reach 100% coverage

[skip ci]

Update PyPI links in readme.

Fix typo in test_persistence.py

Do this everywhere for simplicity. It slows PyPy down by 3x, but that should still only be around 4s.

There are many fewer 'no cover' lines now.

Remove the call to copy_reg.constructor---it doesn't do anything anymore except check that the object is callable, there is no registry.

Also make timestamp support PURE_PYTHON.

Remove the old workaround for a broken __cmp__.

Two of the tests for _p_serial weren't catching what we thought they were.

Deleted _makeOctets since we have native literals everywhere.

Two bug fixes for pure-Python mode: deleting _p_oid and deleting *any* _p attribute.

Test the docs on Python 2 and 3 using manuel. This gets a lot of coverage of tests.attrhooks, tests.cucumbers and tests.utils.

Build CFFI modules the modern way.

Switch to zope.testrunner from 'python setup.py test' and 'nose'.

Both are essentially deprecated.

This is a step in producing usable coverage numbers.

(Also remove cp33 from the manylinux build. This was missed in https://github.com/zopefoundation/persistent/pull/74)

Add support for Python 3.7 and drop 3.3

[skip ci]