Error "expected string" for None in YAML config

[jaraco]

In the docs for yaml config, it says that the default for a requirements_file is None. I assumed None meant a Python None or a null value, so I used the YAML syntax of null for the value, but that triggered this build failure.

It's unclear from the docs how one might specify the default value for a setting. Here's what I recommend:

• Docs for YAML config should specify defaults and example values in YAML syntax (i.e. null for null value).
• RTD should allow for null values (YAML null or Python None) in config where appropriate.
• RTD should use native types in YAML config and not encoded values (i.e. literal None string in YAML).

But most essentially, it should be possible to specify the default value for a given setting. I don't believe that's possible given the current implementation (though admittedly, I haven't looked at the implementation itself).

[stsewd]

I'm solving this issue on the rtd-build repo readthedocs/readthedocs-build#38