* we've been getting reports of new Python 3.7
patch releases from Windows store causing SciPy DLL
load issues: scipy/scipy#11826

* so, extend the `_distributor_init.py` machinery usage
to include Python 3.7 and up for our wheels

* simplify the appveyor `matplotlib` install now that
there are stable releases available for Python `3.8`

* fix warnings in `.travis.yml` based on feedback from
https://config.travis-ci.com/explore

* bump distro to `bionic` since NumPy did this recently too

* attempting to restore Travis CI runs to our wheels
build matrix

* try using `[System.Version]` for more robust
Python version comparisons in powershell

* update minimum NumPy version to `1.14.5`

* the distutils override for appveyor/Windows
builds now only happens for Python < 3.7, since
we have now added the `_distributor_init.py`
for Python 3.7 (not just Python 3.8)

* try using newer NumPy version for Windows Python 3.7
builds, to see if this helps `check_installed_package.py`

…on failure.

* try to solve DLL resolution issues for older
(32-bit) Python versions by temporarily moving into the
path where they are stored prior to load attempts
in `_distributor_init.py`

This reverts commit 46e69e1.

* remove some debug prints

* revert some debug changes

* use `_distributor_init.py` with Python 3.6+
(all supported Python versions)

* wrap the `_distributor_init.py` `os.chdir` code in a
`try .. finally` block to ensure restoration of the working
directory if something fails

* substantially expand the comments describing the rationale
for the working directory changes in `_distributor_init.py`

* sync multibuild submodule to latest `devel` branch
checkout in attempt to deal with Travis CI failures
that just started appearing