Bump urllib3 from 1.25.5 to 1.25.6

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.25.5 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@1.25.5...1.25.6)

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

requirements.txt

@@ -12,7 +12,7 @@ beautifulsoup4==4.8.0
 httplib2<0.13.2
 hyperlink==19.0.0
 idna>=2.5,<2.9
-urllib3==1.25.5
+urllib3==1.25.6
 Paver==1.3.4
 python-slugify==3.0.4
 decorator==4.4.0