Jaraco.develop

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* Removed many crufty modules and trimmed dependencies.

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* macos-build-python now checks that dependencies are installed.

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* Improve reliability of macOS build with reference to xz.

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* Require Python 3.6 or later.
* Removed 'make-namespace-package' command associated
functionality in ``namespace`` module.
* Removed Bitbucket-related functionality. Nobody is going
to need that again.
* Removed Github module. Use `hub <https://hub.github.com/>`_
command.
* Removed selenium code that is old and with limited utility
except on Windows.
* Removed all other command-line scripts.

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Switch to `pkgutil namespace technique
<https://packaging.python.org/guides/packaging-namespace-packages/#pkgutil-style-namespace-packages>`_
for the ``jaraco`` namespace.

Drop support for Python 3.5.

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Updated github to expect a token instead of username/password
in the keyring.