Fix readthedocs build (#261)

- [x] Closes #254 (insert issue number)
- [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
- [x] Added an entry to the CHANGES file

Readthedocs are still not building. I would like to test this PR on RTD
before merging it in since it's taking a few tries to get this working.
It's possible to do this test on a side branch like this while keeping
the docs version hidden from the public.

@lebigot could you please make me and the other uncertainties
maintainers to be maintainers on the uncertainties readthedocs project?

I believe this can be done with something along the lines of
- Go to readthdocs account
- Go into the uncertainties package
- Go to "Admin"
- Go to tab for "Maintainers" 
- Add maintainers by user name. I can be added under the username
jagerber48. If you add me I would be happy to add the others.

.readthedocs.yml

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

CHANGES.rst

@@ -1,6 +1,13 @@
 Change Log
 ===================
 
+Unreleased
+----------
+
+Fixes:
+
+- fix `readthedocs` configuration so that the build passes (#254)
+
 3.2.2 2024-July-08
 -----------------------
 

doc/requirements.txt

@@ -0,0 +1 @@
+.[all]