.readthedocs.yaml: use pip instead of apt to install sphinxcontrib.blockdiag

MaxKellermann · MaxKellermann · commit 1a531cd8ad23 · 2022-01-31T08:53:12.000+01:00
apt did not work:

 ModuleNotFoundError: No module named 'sphinxcontrib.blockdiag'

... but I don't know why.  I don't know what I'm doing, sigh.
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
@@ -1,5 +1,5 @@
 version: 2
 
-build:
-  apt_packages:
-    - python3-sphinxcontrib.blockdiag
+python:
+  install:
+    - requirements: doc/requirements.txt
diff --git a/doc/requirements.txt b/doc/requirements.txt
@@ -0,0 +1 @@
+sphinxcontrib-blockdiag
