req: Remove now useless extra dep of isort from dev dependencies

The "pyproject" extra dependency has been removed from isort 5.0.0,
which has been released in July 2020. It caused a warning with when a
more recent version was installed:

    $ pip install -r requirements_dev.txt
    [...]
    WARNING: isort 5.13.2 does not provide the extra 'pyproject'
    [...]

requirements_dev.txt

@@ -10,7 +10,7 @@ sphinx_rtd_theme
 
 # Quality
 check-manifest
-isort[pyproject]
+isort
 pylint
 pylint_strict_informational
 twine