setup: fix Python 3.6 build

Even though Python 3.6 is EOL we still want to support it because Ubuntu
18.04 LTS is still supported. Work around the build problems by staying
on the last supported setuptools_scm version.

setup.py

@@ -56,7 +56,8 @@ class build(build_orig):
 
 # Installation time dependencies only needed by setup.py
 setup_requires = [
-    'setuptools_scm',   # automatically get package version
+    'setuptools_scm<7',   # automatically get package version
+                          # TODO: remove constraint when Python 3.7 is required
 ]
 
 # Stub class that acts as a proxy for the real sphinx.setup_command.BuildDoc