Bump urllib3 from 1.24.3 to 1.26.18 in /util/pingdom

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.24.3 to 1.26.18.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](urllib3/urllib3@1.24.3...1.26.18)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-type: direct:production
...

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

util/pingdom/requirements.txt

@@ -11,4 +11,4 @@ idna==2.7                 # via requests
 pyyaml==3.12              # via -r requirements/pingdom.in
 requests==2.20.0          # via -r requirements/pingdom.in
 six==1.14.0               # via -r requirements/pingdom.in
-urllib3==1.24.3           # via requests
+urllib3==1.26.18           # via requests