.readthedocs: Add configuration file.

When this file is present, the settings in the online dashboard will be ignored.

This lets us explicitly set the Python version, in this case to 3.8 to match pyproject.toml.

Fixes https://github.com/pybricks/pybricks-api/issues/68.

.readthedocs.yaml

@@ -0,0 +1,22 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+#
+# Settings in the readthedocs online dashboard will be ignored.
+
+# Required
+version: 2
+
+# Build documentation in the doc/main directory with Sphinx
+sphinx:
+   configuration: doc/main/conf.py
+
+# Build your docs in additional formats such as PDF
+formats:
+   - pdf
+
+# Set the version of Python and requirements required to build the docs
+python:
+   version: 3.8
+   install:
+   - requirements: rtd-requirements.txt