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MRG: Add 3.8 tests for linux + mac, make 3.8 the default for mac

Addresses #261 

MRG: Drop old PyPy versions from CI testing on osx

Together with adding a new PyPy version in #266, no need to test PyPy versions that cannot support NumPy even on Python2.7. FWIW, [PyPy 5.4 was released](https://morepypy.blogspot.com/2016/08/pypy2-v54-released-incremental.html) in Aug 2016, [5.3 was released](https://morepypy.blogspot.com/2016/06/pypy2-v53-released-major-c-extension.html) in June 2016 which is 42 months ago, the cutoff as per [NEP 29](https://numpy.org/neps/nep-0029-deprecation_policy.html#detailed-description) for CPython 

MRG: Changed bzip2 download URL

https://download.sourceforge.net/bzip2/ only has 1.0.6, while https://sourceware.org/pub/bzip2/, as found at https://sourceware.org/bzip2/, has 1.0.1 through 1.0.8.

MRG: Allow user to override PLAT for macOS builds

Add ability for user to override `PLAT` for macOS builds. Prior to this `PLAT` was only used for linux builds. 

closes #252 

Fixed typo

MRG: set MB_PYTHON_OSX_VER default to the newest available

Use 10.9 installers where they are available, instead of defaulting to 10.6 installers.

Discussion in #273

Co-Authored-By: Andrew Murray <3112309+radarhere@users.noreply.github.com>

Updated arb to 2.17.0

Fix typo in --enable-threadsafe flag

--enable-unsupported is necesssary for HDF5 to build with both
threadsafety and high level library enabled.

--with-pthreads form HDF5 docs and conda-forge

Thread safety flags copied from conda-forge hdf5-feedstock:
conda-forge/hdf5-feedstock@36304eabd09a68d60a726fd6de4913d38c040917

update SWIG_VERSION from 3.0.12 to 4.0.1

MRG  default macOS ver, brew fix, refactor

This PR does three things:

* Sets the default Python installer / MB_PYTHON_OSX_VER for Python 3.8, so
  users don't have do this in their travis.yml files.
* Adds a brew update fix to work round a syntax error on Homebrew on macOS.
* Refactors our .travis.yml to use MB_PYTHON_VERSION, rather than
  PYTHON_VERSION.  This matches the way that users will write their travis.yml
  files.

MB_PYTHON_VERSION is the standard variable to set the Python version.
Switch to that.  Ensure it is always set for tests.

See: https://github.com/matthew-brett/multibuild/pull/274#issuecomment-547314285

Thanks Matti.

MRG: Build HDF5 with thread safety enabled

Build HDF5 with --enable-thread-safe flag.

This was brought to my attention by @ZanSara over at pytables/pytables#776

It seems a good idea to enable thread safety for the HDF5 library.

This makes a lot of sense, as the conda-forge [hdf5 package is also build](https://github.com/conda-forge/hdf5-feedstock/blob/master/recipe/build.sh) with this flag.

Build HDF5 with --enable-thread-safe flag.

Updated Python versions