python: add 'make check-dev' invocation

This is a *third* way to run the Python tests. Unlike the first two
(check-pipenv, check-tox), this version does not require any specific
interpreter version -- making it a lot easier to tell people to run it
as a quick smoketest prior to submission to GitLab CI.

Summary:

  Checked via GitLab CI:
    - check-pipenv: tests our oldest python & dependencies
    - check-tox: tests newest dependencies on all non-EOL python versions
  Executed only incidentally:
    - check-dev: tests newest dependencies on whichever python version

('make check' does not set up any environment at all, it just runs the
tests in your current environment. All four invocations perform the
exact same tests, just in different execution environments.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Tested-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-id: 20210629214323.1329806-12-jsnow@redhat.com
[Maintainer edit: added .dev-venv/ to .gitignore. --js]
Acked-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Acked-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>