Apply updates from cookiecutter

This automated commit applies the latest updates from our cookiecutters [1] to
this repo.

[1]: https://github.com/hypothesis/cookiecutters

tox.ini

@@ -19,7 +19,6 @@ setenv =
     dev: NEW_RELIC_APP_NAME = {env:NEW_RELIC_APP_NAME:h-matchers}
     dev: NEW_RELIC_ENVIRONMENT = {env:NEW_RELIC_ENVIRONMENT:dev}
     dev,tests,functests: PYTHONDEVMODE = {env:PYTHONDEVMODE:1}
-    tests,functests: PYTEST_PLUGINS = tests.pytest_plugins.factory_boy
     tests: COVERAGE_FILE = {env:COVERAGE_FILE:.coverage.{envname}}
 passenv =
     HOME
@@ -37,6 +36,7 @@ deps =
     lint: pycodestyle
     lint,tests: pytest-mock
     lint,tests,functests: pytest
+    lint,tests,functests: h-testkit
     tests: pytest-cov
     coverage: coverage[toml]
     lint,tests,functests: factory-boy