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Add tests for fill of pypy versions, give full versions for latest.

This changes how the the Python implementation is selected.  It's now
primarily based on enviroment variables.

Uses the PYPY_VERSION enviroment variable to determine the version of PyPy to
install.  An example has been added to the matrix in the readme.

Dashes were missing from untar on MacOS.

Redirect fill_pyver errors to stderr, instead of returning them as the
result.

pip will copy the directory it installs from before running the install
/ wheel build.  If the directory is a submodule, then the copy breaks
the connection with the containing project, so git commands to get the
version will fail.

Fix is to re-create the .git directory for submodules.

Version of fix in https://github.com/matthew-brett/multibuild/pull/12
with thanks to Hassan Kibirige.

* https://github.com/direnv/direnv/issues/210
* https://github.com/travis-ci/travis-ci/issues/6522

[skip ci]

library_builders.sh script in fact needs common_utils.sh.  Source that
script, and put a guard in common_utils.sh to prevent it being sourced
more than once.

For getting versions from versions with suffixes.

Always load library builders (for OSX as well as Manylinux).

Useful for scripts overriding "build_wheel" function.

Make function to download and unpack archives.

This allows caching the archives in the `archives` directory, either by
committing to the git repo, or when doing repeated builds of the same
library.

Slight generalization of wheel building for matplotlib case where
versioneer gets confused.

Make configuration more general.

Move responsibility for source futzing back to the travis.yml file,
because we are now doing only one build per job.