Attempt to fix RTD build

Mock petsc4py imports to avoid building things on RTD, and fix the
docutils version to something that works with m2r2

.readthedocs.yaml

@@ -3,16 +3,11 @@ build:
   os: ubuntu-22.04
   apt_packages:
   - libopenmpi-dev
-  - libpetsc-real3.15-dev
   tools:
     python: "3.11"
 python:
   install:
-  # petsc4py doesn't explicitly declare its dependencies
-  # they need to be installed beforehand
-  - requirements: docs/requirements-petsc4py.txt
-  # astrovascpy doesn't explicitly declare its dependence on petsc4py
-  # it needs to be installed beforehand
+  # Markdown conversion requires a particular version of docutils
   - requirements: docs/requirements.txt
   - method: pip
     path: .

docs/requirements.txt

@@ -1 +1 @@
-petsc4py==3.15
+docutils==0.19

docs/source/conf.py

@@ -10,14 +10,14 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 
-from pkg_resources import get_distribution
+from importlib.metadata import version as get_version
 
 # -- Project information -----------------------------------------------------
 
 project = "AstroVascPy"
 
 # The short X.Y version
-version = get_distribution(project).version
+version = get_version(project)
 
 # The full version, including alpha/beta/rc tags
 release = version
@@ -79,6 +79,7 @@
     "members": True,
     "show-inheritance": True,
 }
+autodoc_mock_imports = ["petsc4py"]
 
 intersphinx_mapping = {
     # Uncomment these lines if you need them