Bump testfixtures from 6.10.3 to 6.15.0

Bumps [testfixtures](https://github.com/Simplistix/testfixtures) from 6.10.3 to 6.15.0.
- [Release notes](https://github.com/Simplistix/testfixtures/releases)
- [Changelog](https://github.com/Simplistix/testfixtures/blob/master/CHANGELOG.rst)
- [Commits](simplistix/testfixtures@6.10.3...6.15.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

test-requirements.txt

@@ -1,6 +1,6 @@
 argon2-cffi>=16.1.0
 factory-boy<=2.12.0
 httmock==1.3.0
-testfixtures==6.10.3
+testfixtures==6.15.0
 tox
 asynctest