Fix docs deployment failing on 3.6 branch

[krassowski]

References

Failure:

Problem in your project's configuration. Invalid configuration option "build.os": build not found

was seen in #15395

Code changes

Adds build.os and other relevant build config.

User-facing changes

None

Backwards-incompatible changes

None

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[krassowski]

It now fails later on with a different traceback:

python -mvirtualenv $READTHEDOCS_VIRTUALENV_PATH
usage: virtualenv [--version] [--with-traceback] [-v | -q] [--read-only-app-data] [--app-data APP_DATA] [--reset-app-data] [--upgrade-embed-wheels] [--discovery {builtin}] [-p py] [--try-first-with py_exe]
                  [--creator {builtin,cpython3-posix,venv}] [--seeder {app-data,pip}] [--no-seed] [--activators comma_sep_list] [--clear] [--no-vcs-ignore] [--system-site-packages] [--symlinks | --copies] [--no-download | --download]
                  [--extra-search-dir d [d ...]] [--pip version] [--setuptools version] [--wheel version] [--no-pip] [--no-setuptools] [--no-wheel] [--no-periodic-update] [--symlink-app-data] [--prompt prompt] [-h]
                  dest
SystemExit: 2
virtualenv: error: the following arguments are required: dest

Bumping python version did not solve it.

[krassowski]

All right, this works:

[jtpio]

Thanks!