Fix #140 by turning OverflowError into TypeError.

This only showed up on Windows (because it follows the LLP64 data model). See https://ci.appveyor.com/project/mgedmin/btrees/builds/31998447/job/m3xs7q1d8b4u5u3b

Fingers crossed for 32-bit builds on Windows.

Ensure resolution order for all objects is consistent

Fixes #137.

Fixed tox 'docs' environment

Fix __all__

Python 3: Stop using undocumented PyLong_AS_LONG.

Add unit tests for it and run the documentation tests that originaly caught the problem in CI.

Fixes #132

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.