Bump urllib3 from 1.24.2 to 1.25.3

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.24.2 to 1.25.3. - [Release notes](https://github.com/urllib3/urllib3/releases) - [Changelog](https://github.com/urllib3/urllib3/blob/master/CHANGES.rst) - [Commits](urllib3/urllib3@1.24.2...1.25.3) Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -64,7 +64,7 @@ terminaltables==3.1.0
 testpath==0.3
 tornado==4.5.1
 traitlets==4.3.2
-urllib3==1.24.2
+urllib3==1.25.3
 virtualenv==15.1.0
 wcwidth==0.1.7
 webencodings==0.5.1