We do need Twisted 14, not 15: we use internal Twisted things that have been removed in 15.

setup.py

@@ -33,7 +33,7 @@ setup(
     install_requires=[
         "syutil==0.0.2",
         "matrix_angular_sdk==0.6.0",
-        "Twisted>=14.0.0",
+        "Twisted==14.0.2",
         "service_identity>=1.0.0",
         "pyopenssl>=0.14",
         "pyyaml",

synapse/python_dependencies.py

@@ -6,7 +6,7 @@ logger = logging.getLogger(__name__)
 REQUIREMENTS = {
     "syutil==0.0.2": ["syutil"],
     "matrix_angular_sdk==0.6.0": ["syweb>=0.6.0"],
-    "Twisted>=14.0.0": ["twisted>=14.0.0"],
+    "Twisted==14.0.2": ["twisted==14.0.2"],
     "service_identity>=1.0.0": ["service_identity>=1.0.0"],
     "pyopenssl>=0.14": ["OpenSSL>=0.14"],
     "pyyaml": ["yaml"],