Comment reminding to replace extras with depgroups

Our use of project.optional-dependencies for nox test environment
package lists is effectively an abuse of that feature, which is
intended to be more user-facing. There is finally a solution agreed
upon in PEP 735 and supported in unreleased pip under development,
slated for inclusion in pip 25.1 (which will require Python 3.9):

https://github.com/pypa/pip/commit/e930fee

Until we can rely on that, add code comments to pyproject.toml and
noxfile.py as a reminder.

While we're here, also correct a TODO comment about license
expressions, the fix for which will also require a minimum of Python
3.9 to work.

Change-Id: I6f38e9a48b06fcea9ad6adca195bb4204cd42a64

noxfile.py

@@ -11,12 +11,14 @@ nox.options.sessions = ["tests-3", "linters"]
 # version using --force-python.
 @nox.session(python="3")
 def linters(session):
+    # TODO: switch to a dependency group after Python 3.8 support is dropped
     session.install(".[test-linters]")
     session.run("flake8")
 
 
 @nox.session(python="3")
 def docs(session):
+    # TODO: switch to a dependency group after Python 3.8 support is dropped
     session.install(".[build-docs]")
     session.run(
         "sphinx-build", "-W",
@@ -28,6 +30,7 @@ def docs(session):
 
 @nox.session(python="3")
 def venv(session):
+    # TODO: switch to a dependency group after Python 3.8 support is dropped
     session.install("-e", ".[test-unit]")
     session.run(*session.posargs)
 
@@ -35,6 +38,7 @@ def venv(session):
 # This will attempt to run python3 tests by default.
 @nox.session(python=["3"])
 def tests(session):
+    # TODO: switch to a dependency group after Python 3.8 support is dropped
     session.install("-e", ".[test-unit]")
     session.run("stestr", "run", *session.posargs)
     session.run("stestr", "slowest")
@@ -42,6 +46,7 @@ def tests(session):
 
 @nox.session(python="3")
 def cover(session):
+    # TODO: switch to a dependency group after Python 3.8 support is dropped
     session.install("-e", ".[test-cover]")
     session.env["PYTHON"] = "coverage run --source bindep --parallel-mode"
     session.run("stestr", "run", *session.posargs)

pyproject.toml

@@ -45,8 +45,8 @@ keywords = [
     "distro",
     "package",
 ]
-# TODO: Add 'license-expression = "Apache-2.0"' here and drop the License
+# TODO: Add 'license = "Apache-2.0"' here and drop the License classifier above
-# classifier once https://github.com/pypa/setuptools/issues/4629 is resolved.
+# once support for Python 3.8 is dropped (requires SetupTools>=77).
 maintainers = [
     {email = "service-discuss@lists.opendev.org"},
     {name = "OpenDev Contributors"},
@@ -55,6 +55,8 @@ name = "bindep"
 readme = {charset = "UTF-8", content-type = "text/x-rst", file = "README.rst"}
 requires-python = ">=3.7"
 
+# TODO: replace these with PEP 735 compliant dependency groups once support for
+# Python 3.8 is dropped (requires pip>=25.1).
 [project.optional-dependencies]
 build-docs = [
     # needed because we use autodoc to document our tests in the dev docs