Fix for #16

Thanks!

see https://github.com/zopefoundation/ZODB/issues/16

see https://github.com/zopefoundation/ZODB/issues/16

For pypy, set the persistent_id attribute of the pickler, since it uses a python implementation even in Py2

This adds 'repozo --verify' to check your backup integrity

Extract size/checksum computation logic into separate functions for
gzipped and non-gzipped files.

Could be used as a workaround for https://bugs.launchpad.net/zodb/+bug/906057

Should fix http://winbot.zope.org/builders/ZODB_dev%20py_265_win32/builds/1086/steps/test/logs/stdio

Should fix a test failure on Windows:
http://winbot.zope.org/builders/ZODB_dev%20py_265_win32/builds/1083/steps/test/logs/stdio

Also, avoid shadowing the 'file' builtin / alias with locals.

- Allow protocol 3 under Python 3.

- Do not stori bytes as strings under Python 3.

Fixes issue #4.

That doesn't work, just causes all but one of the functional test layers
to be silently skipped (thanks to zopefoundation/zope.testrunner#5 and
zopefoundation/zope.testrunner#6).

I don't know why zope.testrunner quits after the first layer.  I don't
want to know.  Let's try running tests exactly the same way as tox.ini.