It's quite hard to get python2 to exit gracefully when the code was
intended for python3, because the interpreter will SyntaxError before
running a single line of code. Thankfully, the pytest developers put a
version check in their .ini config, so that should gracefully handle
both wrong-pytest-version and wrong-python-version.
Also document the woes of trying to run the pytest version shipped by
e.g. Debian or Ubuntu.
