chore: add an isort tox environment and run isort in CI

  * Add an isort tox environment
  * Run the isort tox environment using --check in the Github CI

https://pycqa.github.io/isort/

.github/workflows/lint.yml

@@ -32,3 +32,5 @@ jobs:
         run: tox -e pep8
       - name: Run mypy static typing checker (http://mypy-lang.org/)
         run: tox -e mypy
+      - name: Run isort import order checker (https://pycqa.github.io/isort/)
+        run: tox -e isort -- --check

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 1.6
 skipsdist = True
-envlist = py39,py38,py37,py36,pep8,black,twine-check,mypy
+envlist = py39,py38,py37,py36,pep8,black,twine-check,mypy,isort
 
 [testenv]
 passenv = GITLAB_IMAGE GITLAB_TAG
@@ -31,6 +31,14 @@ deps = -r{toxinidir}/requirements.txt
 commands =
   black {posargs} .
 
+[testenv:isort]
+basepython = python3
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+       isort
+commands =
+  isort --dont-order-by-type {posargs} {toxinidir}
+
 [testenv:twine-check]
 basepython = python3
 deps = -r{toxinidir}/requirements.txt
@@ -60,6 +68,11 @@ ignore = E203,E501,W503
 per-file-ignores =
     gitlab/v4/objects/__init__.py:F401,F403
 
+[isort]
+profile = black
+multi_line_output = 3
+force_sort_within_sections = True
+
 [testenv:docs]
 deps = -r{toxinidir}/rtd-requirements.txt
 commands = python setup.py build_sphinx