See if this helps

setup.cfg

@@ -36,8 +36,10 @@ install_requires =
     eliot
     aniso8601
     python-challenge-bypass-ristretto
-    # Inherit our Twisted dependency from tahoe-lafs so we don't accidentally
-    # get the extras wrong here and break stuff.
+    # The pip resolver sometimes finds treq's dependencies first and these are
+    # incompatible with Tahoe-LAFS'.  So duplicate them here (the ones that
+    # have been observed to cause problems).
+    Twisted[tls,conch]>=18.4.0
     tahoe-lafs==1.14.0
     treq