chore(deps): update dependency python3-saml to >=1.15,<1.16

Author: renovate[bot]

sentry/optional-requirements.txt

@@ -2,4 +2,4 @@ GeoIP==1.3.2
 google-cloud-bigtable>=0.32.1,<0.33.0
 google-cloud-pubsub>=0.35.4,<0.36.0
 google-cloud-storage>=1.13.2,<1.14
-python3-saml>=1.4.0,<1.5
+python3-saml>=1.15,<1.16