have readthedocs install with , drop requirements files (#255)

- [x] Closes #254
- [x] Executed `pre-commit run --all-files` with no errors
- [x] The change is fully covered by automated unit tests
- [x] Documented in docs/ as appropriate
- [ ] Added an entry to the CHANGES file


This changes the readthedocs config to install uncertainties and
dependencies with `pip install ".[all]"`.

Author: newville

.readthedocs.yml

@@ -29,4 +29,4 @@ sphinx:
 # See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 python:
   install:
-  - requirements: doc/requirements.txt
+  - pip install ".[all]"