Python 3: Stop using undocumented PyLong_AS_LONG.

Since https://github.com/zopefoundation/persistent/issues/125 this would result in compiler warnings.

No change note, this is an internal change.

Fixes #131

Added internal documentation links

Remove most all the boilerplate required to add new trees

Developper documentation improvements

Generate the Python modules and test modules based off a description
of the datatype.

Several tests that were previously only used for a specific classes
are now generalized and added to most trees, such as testing
non-compliant keys and testing overflow for bounded values.

Also always raise ``OverflowError`` for integer keys/values out of range.

And some other small tweaks to the documentation to make cross-references work and things generally look good.

test__UUBTree.py is complete and passing on Python 3

Fix it.

Also it's possible to statically know the expected value of using64bits for all included tree modules, so we don't need the (unexecuted) branch that creates (unused) test code in the *I* test modules.

And other small tweaks to bring test coverage up. Turns out there were several test methods being needlessly skipped.

adds python 3.8 on macos to the travis build.

Fix Python 3.5 MacOS wheel upload

Python 3.5 on Mac OS doesn't support TLS 1.2, which makes twine upload
fail, but we can use system Python 2.7 for twine upload, which works.

Also brings the rest of the Travis CI configuration closer in line to
the rest of the packages updated for
https://github.com/zopefoundation/meta/issues/11

Build using newer manylinux image.

And update macOS python to 2.7.17 to try to fix uploads.

Restructure Travis to always build manylinux wheels

This PR makes it so we _always_ build manylinux wheels, but only upload
them for git tags.

The diff is messy, for which I apologize.  I had to reformat the YAML to
understand it.  Highlights:

- Moved regular Linux Python builds outside build matrix, for
  conciseness (unfortunately this means 3.6 builds are now done twice:
  once without WITH_COVERAGE and once with)

- Move twine upload from .manylinux.sh into .travis.yml's after_success,
  next to the twine upload used for Mac OS builds

- This means Travis can no longer do an 'exit 0' in before_install, so I
  had to override the install and script sections too

- Move 'docker pull' into the install section so it's not so very empty

- Unify the two calls to twine upload to provide the twine password via
  environment variables instead of command-line or configuration files

- Put related commands inside a single multiline if statement instead of
  repeating the conditions

Add support for Python 3.8

I'm skipping the MacOS build on Travis because 3.8 is not available yet
from https://github.com/MacPython/terryfy

Some documentation love