Stop configuring 'install_command'

Currently, we are overriding 'install_command' to use 'pip'. This is
considered poor behavior and 'python -m pip' should be used instead.

https://snarky.ca/why-you-should-use-python-m-pip/

It turns out that this is the the default value provided by tox:

https://tox.readthedocs.io/en/latest/config.html#conf-install_command

So we can remove the line and simply use the default value.

Change-Id: I591475707205732e68e3e68231c417acf672277b

{{cookiecutter.repo_name}}/tox.ini

@@ -5,9 +5,7 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install {opts} {packages}
 setenv =
-   VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
    OS_STDOUT_CAPTURE=1
    OS_STDERR_CAPTURE=1