Bump urllib3 from 1.24.2 to 1.25.6

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.24.2 to 1.25.6.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/master/CHANGES.rst)
- [Commits](urllib3/urllib3@1.24.2...1.25.6)

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

Author: dependabot[bot]

requirements.txt

@@ -32,7 +32,7 @@ pytest-cov==2.5.1
 PyYAML==5.1
 requests==2.18.4
 six==1.11.0
-urllib3==1.24.2
+urllib3==1.25.6
 virtualenv==15.2.0
 wcwidth==0.1.7
 Werkzeug==0.15.3