Pin packaging while building docs

Packaging >=22 no longer supports 'latest' versions, which is the version that is assigned when building the docs.

https://github.com/pytest-dev/pytest/pull/10578#issuecomment-1348249045

Also first install pytest, then `docs/en/requirements.txt`:

Installing the requirements last will honor pins of libraries
shared between pytest and the docs build.

.readthedocs.yml

@@ -2,9 +2,12 @@ version: 2
 
 python:
    install:
-      - requirements: doc/en/requirements.txt
+     # Install pytest first, then doc/en/requirements.txt.
-      - method: pip
+     # This order is important to honor any pins in doc/en/requirements.txt
-        path: .
+     # when the pinned library is also a dependency of pytest.
+     - method: pip
+       path: .
+     - requirements: doc/en/requirements.txt
 
 build:
   os: ubuntu-20.04

doc/en/requirements.txt

@@ -5,3 +5,7 @@ sphinx-removed-in>=0.2.0
 sphinx>=5,<6
 sphinxcontrib-trio
 sphinxcontrib-svg2pdfconverter
+# Pin packaging because it no longer handles 'latest' version, which
+# is the version that is assigned to the docs.
+# See https://github.com/pytest-dev/pytest/pull/10578#issuecomment-1348249045.
+packaging <22