Bump minimum PyYAML to 3.13. (#14720)

PyYAML 3.13 fixes some issues with Python 3.7 compatibility
and was released in 2018.

changelog.d/14720.misc

@@ -0,0 +1 @@
+Bump minimum PyYAML to 3.13.

pyproject.toml

@@ -136,7 +136,7 @@ Twisted = {extras = ["tls"], version = ">=18.9.0"}
 treq = ">=15.1"
 # Twisted has required pyopenssl 16.0 since about Twisted 16.6.
 pyOpenSSL = ">=16.0.0"
-PyYAML = ">=3.11"
+PyYAML = ">=3.13"
 pyasn1 = ">=0.1.9"
 pyasn1-modules = ">=0.0.7"
 bcrypt = ">=3.1.7"