Include '--index-url' argument for the pip install command for Test PyPI (#4941)
Include '--index-url' argument for the pip install command for Test PyPI (#4941)

* Include '--index-url' argument for the pip install command for Test PyPI

The template logic for a package's pip installation command has been
moved to a jinja macro called `pip_command`. This macro furthermore
determines whether or not to add the Test PyPI's `--index-url` based on
the existence of the *testPyPI* variable.

* Shorten text by using `-i`

pypa/warehouse,dstufft/warehouse,dstufft/warehouse,pypa/warehouse,pypa/warehouse,dstufft/warehouse,pypa/warehouse,dstufft/warehouse