Wrap black with script that skips it if Python version < 3.6.

scripts/black

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+set -e
+
+python_version=$(python --version)
+
+if [[ "$python_version" =~ "Python 3.5." ]]; then
+    echo "Skipping black due to lack of support for $python_version."
+elif [[ "$python_version" =~ "Python 3.4." ]]; then
+    echo "Skipping black due to lack of support for $python_version."
+else
+    black --skip-string-normalization --line-length 100 --check .
+fi

tox.ini

@@ -5,10 +5,11 @@ skipsdist = True
 [testenv]
 usedevelop = True
 deps = -rtest_requirements.txt
+whitelist_externals = bash
 commands =
     py.test --cov-report term-missing:skip-covered --cov=borgmatic --ignore=tests/end-to-end \
         tests []
-    - black --skip-string-normalization --line-length 100 --check .
+    bash scripts/black
     flake8 .
 
 [testenv:black]