Use Python 3.6.2 on MacOS

3.6.1 broke ABI, 3.6.2 fixed it.  More details at
https://bugs.python.org/issue29943.

It seems that 3.6.0 has a bug that causes
https://github.com/nedbat/coveragepy/issues/703 ("Couldn't use data file
'/Users/travis/build/zopefoundation/persistent/.coverage': Safety level
may not be changed inside a transaction" and a failed build from
'coverage run').

terrif now supports py3.8 builds on mac

Build with manylinux2010 and use Python 2.7.17 to build on mac in order to upload.

Fix manylinux wheel builds and build them always

I though pip was smart, but pip is stupid and it ignores version
constraints declared by auditwheel when you upgrade a bunch of packages.

This also reformats the YAML, for which I apologize, but it was hard to
work on it without doing that.

It drops dist: xenial which has become the default.

It renames matrix: to jobs: because that's the preferres spelling
according to Travis CI docs.

It adds names to all the special build jobs.

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

It moves docker pull and twine upload out of .manylinux.sh and into
.travis.yml.

It unifies twine upload calls by supplifying username/password via
environment variables.

The changes largely mirror those in
https://github.com/zopefoundation/BTrees/pull/115.

Should fix #113.

We can't see if the fix works because wheels are built only for git
tags.

Enable Python 3.8 on Appveyer and also for manylinux builds

Since 3.8 is not available on Appveyor yet, use a script to install it.

+ Test pure versions and docs in TravisCI.

Fix C compilation warning

PRIx64 is defined in <inttypes.h>, not <stdint.h>.

I hate C.

gcc 8.3.0 emits this during python setup.py build:

persistent/cPersistence.c: In function ‘Per_repr’:
persistent/cPersistence.c:1481:44: warning: format ‘%llx’ expects argument of type ‘long long unsigned int’, but argument 4 has type ‘uint64_t’ {aka ‘long unsigned int’} [-Wformat=]
         snprintf(buf, sizeof(buf) - 1, "%llx", oid_value);
                                         ~~~^   ~~~~~~~~~
                                         %lx

<stdint.h> defines standard macros such as PRId64, PRIu64 and PRIx64 for
printing {u,}int64_t values, so let's use them.

Use Py_ssize_t for length arguments of PyArg_ParseTuple

This API was introduced in Python 2.5 and the old one (using ints) has
been deprecated since Python 3.8.

See #108.

This API was introduced in Python 2.5 and the old one (using ints) has
been deprecated since Python 3.8.

Fixes #108.

Require CFFI on CPython for pure-Python operation.

Fixes #77

Allow sweeping cache without cache_size and with cache_size_bytes only.