Update packaging requirement to support >= 20.4 (e.g. 21.0) (#4362)

pyproject.toml

@@ -43,9 +43,11 @@ html5lib = "^1.0"
 shellingham = "^1.1"
 tomlkit = ">=0.7.0,<1.0.0"
 pexpect = "^4.7.0"
-packaging = "^20.4"
+# Leave unclamped since packaging doesn't follow semantic versioning.
+packaging = ">=20.4"
 # exclude 20.4.5 - 20.4.6 due to https://github.com/pypa/pip/issues/9953
 virtualenv = "(>=20.4.3,<20.4.5 || >=20.4.7)"
+# Leave unclamped since keyring doesn't follow semantic versioning.
 keyring = ">=21.2.0"
 entrypoints = "^0.3"
 importlib-metadata = {version = "^1.6.0", python = "<3.8"}