Try to fix missing myst-parser

.readthedocs.yaml

@@ -0,0 +1,6 @@
+version: 2
+
+sphinx:
+  builder: html
+  configuration: conf.py
+  fail_on_warning: true

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["flit_core >=3.2,<4"]
+requires = ["flit_core >=3.2,<4", "myst-parser"]
 build-backend = "flit_core.buildapi"
 
 [project]