exclude tahoe-lafs 1.15 which causes problems for pip

tahoe-lafs 1.15 depends on attrs<20
something else pulls in attrs 21.2.0
pip fails to resolve the dependencies correctly in this case

setup.cfg

@@ -38,7 +38,7 @@ install_requires =
     # 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, <1.17
+    tahoe-lafs >=1.14, <1.17, !=1.15
     treq
     pyutil