session.env_select.CliEnv: Document and test spaces behaviour (#3208)

It turns out that spaces on either side of environment names in a
comma-separated list are considered not part of the environment
name and removed. This was neither documented nor tested
directly; that's now fixed. (It was and is still also tested in
`config.cli.test_cli_ini.test_ini_exhaustive_parallel_values` as
well.)

The discovery of this was documented in issue #3207.

src/tox/session/env_select.py

@@ -27,13 +27,15 @@
 
 
 class CliEnv:  # noqa: PLW1641
-    """A `CliEnv` is the user's selection of tox test environments, usually supplied via the ``-e`` command-line
-    option. It may be treated as a sequence if it's not a "default" or "all" selection.
+    """A `CliEnv` is the user's selection of tox test environments, usually supplied via the ``-e`` command-line option
+    or in a TOML file (typically ``env_list`` in ``tox.ini``). It may be treated as a sequence if it's not a "default"
+    or "all" selection.
 
     It is in one of three forms:
 
     - A list of specific environments, instantiated with a string that is a comma-separated list of the environment
-      names. As a sequence this will be a sequence of those names.
+      names. (These may have spaces on either side of the commas which are removed.) As a sequence this will be a
+      sequence of those names.
 
     - "ALL" which is all environments defined by the tox configuration. This is instantiated with ``ALL`` either
       alone or as any element of a comma-separated list; any other environment names are ignored. `is_all()` will be

tests/session/test_env_select.py

@@ -23,6 +23,7 @@
         ("", (), False, True),
         ("a1", ("a1",), False, False),
         ("a1,b2,c3", ("a1", "b2", "c3"), False, False),
+        (" a1, b2 ,  c3  ", ("a1", "b2", "c3"), False, False),
         #   If the user gives "ALL" as any envname, this becomes an "is_all" and other envnames are ignored.
         ("ALL", (), True, False),
         ("a1,ALL,b2", (), True, False),