Bump readme-renderer[md] from 25.0 to 26.0

Bumps [readme-renderer[md]](https://github.com/pypa/readme_renderer) from 25.0 to 26.0.
- [Release notes](https://github.com/pypa/readme_renderer/releases)
- [Changelog](https://github.com/pypa/readme_renderer/blob/master/CHANGES.rst)
- [Commits](https://github.com/pypa/readme_renderer/compare/25.0...26.0)

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

requirements-dev.txt

@@ -3,6 +3,6 @@ mock
 nose
 pycodestyle==2.5.0
 pyflakes==2.1.1
-readme_renderer[md]==25.0
+readme_renderer[md]==26.0
 requests_mock
 twine==1.15.0