python: add Makefile for some common tasks

python: add Makefile for some common tasks

Add "make venv" to create the pipenv-managed virtual environment that
contains our explicitly pinned dependencies.

Add "make check" to run the python linters [in the host execution
environment].

Add "make venv-check" which combines the above two: create/update the
venv, then run the linters in that explicitly managed environment.

Add "make develop" which canonizes the runes needed to get both the
linting pre-requisites (the "[devel]" part), and the editable
live-install (the "-e" part) of these python libraries.

make clean: delete miscellaneous python packaging output possibly
created by pipenv, pip, or other python packaging utilities

make distclean: delete the above, the .venv, and the editable "qemu"
package forwarder (qemu.egg-info) if there is one.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20210527211715.394144-29-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>