A development convenience to pass arguments like -v -x and others.

See:
https://tox.wiki/en/latest/config.html#substitutions-for-positional-arguments-in-commands

... and remove `amazon` from "missing distributions" test cases list

> closes #252

> closes #320

This allows executing the file as a standalone script.

Previously, without the shebang, trying to execute results in the
following:

    $ ./distro.py
    ./distro.py: line 28: platform.linux_distribution: command not found
    ./distro.py: line 28: platform.dist: command not found
    ./distro.py: line 28: Python: command not found
    ./distro.py: command substitution: line 29: syntax error near unexpected token `newline'
    ./distro.py: command substitution: line 29: `<https://bugs.python.org/issue1322>'
    ...

Now:

    $ ./distro.py
    Name: Fedora 34 (Workstation Edition)
    Version: 34
    Codename:

distro#230 introduced proper support for os-release `VERSION_CODENAME`
attribute but broke compatibility shim with (now defunct)
`platform.linux_distribution` as the third tuple member (codename) were
not taken from `VERSION` os-release attribute anymore.

Closes #238.

`subprocess.CalledProcessError` exceptions are now caught since 9260b7b

> closes #303

…#333)