Fixing doc build

The error during build was:
```
WARNING: autodoc: failed to import module 'pytds'; the following exception was raised:
No module named 'six'
WARNING: autodoc: failed to import module 'login' from module 'pytds'; the following exception was raised:
No module named 'six'
```

.readthedocs.yaml

@@ -17,6 +17,6 @@ sphinx:
 
 # We recommend specifying your dependencies to enable reproducible builds:
 # https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
-# python:
-#   install:
-#   - requirements: docs/requirements.txt
+python:
+  install:
+  - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1 @@
+six>=1.4.1