try to avoid the setuptools_scm trap

setup.cfg

@@ -42,6 +42,13 @@ install_requires =
     # have been observed to cause problems).
     Twisted[tls,conch] >= 19.10.0
     tahoe-lafs >=1.17,<1.18
+
+    # Tahoe-LAFS 1.17 adds a cbor2 dependency; cbor2 has a setuptools_scm
+    # dependency; setuptools_scm has dropped Python 2 support without
+    # declaring it in their metadata.  Pin setuptools_scm to an older version
+    # here to avoid getting a broken version.
+    setuptools_scm < 6
+
     treq
     pyutil
     prometheus-client