chore(deps): bump urllib3 from 1.26.17 to 1.26.18 in /.kokoro

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.17 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@1.26.17...1.26.18)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-type: indirect
...

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

.kokoro/requirements.txt

@@ -467,9 +467,9 @@ typing-extensions==4.4.0 \
     --hash=sha256:1511434bb92bf8dd198c12b1cc812e800d4181cfcb867674e0f8279cc93087aa \
     --hash=sha256:16fa4864408f655d35ec496218b85f79b3437c829e93320c7c9215ccfd92489e
     # via -r requirements.in
-urllib3==1.26.17 \
-    --hash=sha256:24d6a242c28d29af46c3fae832c36db3bbebcc533dd1bb549172cd739c82df21 \
-    --hash=sha256:94a757d178c9be92ef5539b8840d48dc9cf1b2709c9d6b588232a055c524458b
+urllib3==1.26.18 \
+    --hash=sha256:34b97092d7e0a3a8cf7cd10e386f401b3737364026c45e622aa02903dffe0f07 \
+    --hash=sha256:f8ecc1bba5667413457c529ab955bf8c67b45db799d159066261719e328580a0
     # via
     #   requests
     #   twine