Use RTD v2 config to require Python newer than default 3.7 in v1

https://readthedocs.org/projects/python-humanize/builds/20906653/

.readthedocs.yml

@@ -1,2 +1,19 @@
-python:
-    pip_install: true
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+# Project page: https://readthedocs.org/projects/python-humanize/
+
+version: 2
+
+mkdocs:
+  configuration: mkdocs.yml
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3"
+
+  commands:
+    - pip install -U tox
+    - tox -e docs
+    - mkdir _readthedocs
+    - mv site _readthedocs/html