rtd: move pip dependencies to conda environment

When using conda the requirements in the main RTD yaml are ignored[1] as
it should be managed by conda. Conda docs[2] indicate how to use pip in
a conda environment, so migrate to that.

Commit caba061910 earlier introduced conda
to the RTD build system, but migration of environments was not noticed
at the time. With new furo theme dependency changes the build fails as
those dependencies are not installed for the RTD build.

[1] https://docs.readthedocs.io/en/latest/config-file/v2.html#requirements-file
[2] https://docs.conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#using-pip-in-an-environment

.readthedocs.yaml

@@ -2,10 +2,6 @@ version: 2
 
 python:
   version: 3
-  install:
-    - requirements: requirements/development.txt
-    - method: pip
-      path: .
 
 sphinx:
   builder: html
@@ -13,4 +9,4 @@ sphinx:
   fail_on_warning: false
 
 conda:
-  environment: documentation/environment.yaml
+  environment: documentation/environment.yaml

documentation/environment.yaml

@@ -5,7 +5,10 @@ channels:
 dependencies:
   # doxygen versions available from conda forge are listed at
   # https://anaconda.org/conda-forge/doxygen/files
-  # 
+  #
   # - doxygen=1.9.4 # to specify a specific version of doxygen
   # we'll be using the latest available
   - doxygen
+  - pip
+  - pip:
+    - -r ../requirements/development.txt