fix: Add six to install requirements for yadage-schemas (#77)

yadage-schemas currently (v0.10.6) fails to specify its requirement on
six v1.4.0+ in its install_requires. To safeguard against this,
temporarily add six>=1.4.0 to install_requires in setup.cfg.

setup.cfg

@@ -39,6 +39,7 @@ install_requires =
     jsonschema>=3.0.0
     pyyaml>=5.1  # for parsing CLI options
     yadage-schemas==0.10.6  # lock to yadage
+    six>=1.4.0  #  c.f. https://github.com/yadage/yadage-schemas/issues/35
 
 [options.packages.find]
 where = src