Bump urllib3 from 1.24.2 to 1.25.7

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.24.2 to 1.25.7.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/master/CHANGES.rst)
- [Commits](https://github.com/urllib3/urllib3/compare/1.24.2...1.25.7)

Signed-off-by: dependabot-preview[bot] <[email protected]>

requirements.txt

@@ -22,5 +22,5 @@ requests==2.22.0
 six==1.12.0
 subprocess32==3.5.4; python_version < '3.2'
 texttable==1.6.2
-urllib3==1.24.2; python_version == '3.3'
+urllib3==1.25.7; python_version == '3.3'
 websocket-client==0.57.0