Sort imports with isort

Keep imports alphabetically sorted and their order homogeneous across
Python source files.

The isort project has more feature and is more active than the
flake8-import-order plugin.

Most issues caught were simply import ordering from the same module.
Where imports were purposefully placed out of order, tag with
isort:skip.

.github/workflows/lint.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        tool: [docslint, flake8, mypy, twine]
+        tool: [docslint, flake8, isort, mypy, twine]
 
     steps:
     - uses: actions/checkout@v2