Use a more modern approach to run tests (#1089)

Running tests through `python setup.py test` is deprecated:

> WARNING: Testing via this command is deprecated and will be removed
> in a future version. Users looking for a generic test entry point
> independent of test runner are encouraged to use tox.

I am not in favor of moving back to `tox`, we should simply run tests
using `python -m pytest` (or `make test`) and that's it.

A new extra was added, `dev`, to install development requirements:

    $ python -m pip install --upgrade --editable '.[dev]'

.github/workflows/build.yml

@@ -30,5 +30,5 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
       - run: python -m pip install --upgrade pip setuptools wheel
-      - run: python -m pip install --upgrade --editable .
-      - run: python setup.py test
+      - run: python -m pip install --upgrade '.[dev]'
+      - run: python -m pytest --verbose ./httpie ./tests