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.

1 files changed, 11 insertions, 0 deletions

diff --git a/tox.ini b/tox.ini
index 316655d20..5bb5b4639 100644
--- a/tox.ini
+++ b/tox.ini
@@ -41,6 +41,17 @@ extras =
 commands =
     flake8 {posargs}
 
+[testenv:isort]
+basepython = python3
+description =
+    Run import sorting checks.
+whitelist_externals =
+    isort
+extras =
+    lint
+commands =
+    isort --check-only --diff .
+
 [testenv:coverage]
 basepython = python3
 description =