Arguments to pip-run prior to `--` are used to specify the requirements
to make available, just as arguments to pip install. For example:

	pip-run -r requirements.txt "requests>=2.0"

That will launch python after installing the deps in requirements.txt
and also a late requests. Packages are always installed to a temporary
location and cleaned up when the process exits.

Arguments after `--` are passed to the Python interpreter. So to launch
`script.py`:

	pip-run -- script.py

For simplicity, the `--` may be omitted and Python arguments will
be inferred starting with the first Python file that exists:

	pip-run script.py

To invoke another executable instead of Python, prefix the argument
with a `!`:

	pip-run cowsay -- !cowsay "hello!"

If the `--` is omitted or nothing is passed, the python interpreter
will be launched in interactive mode:

	pip-run
	>>>

For more examples and details, see https://pypi.org/project/pip-run.
