whitespace around a comma rejected in rule lists

[jshwi]

A rule list written with a space after the comma is not parsed as two rules.
_split_comma splits the commandline value and Comment splits a directive's
value, but neither strips the parts, so the second ref carries a leading space
and never matches a message.

The two surfaces then fail differently: the commandline rejects the run
outright, while a directive leaves the unmatched rule enabled and reports
SIG003 against it, so the user is told the wrong thing about a check that
still fires.

Reproducer

t.py:

def f(param1, param2) -> int:
    """Summary."""
    return 0

$ docsig --disable "SIG203, SIG503" t.py
unknown option to disable ' SIG503'
$ echo $?
2

Expected: exit 0, the same as --disable "SIG203,SIG503".

The same list in a directive:

# docsig: disable=SIG203, SIG503
def f(param1, param2) -> int:
    """Summary."""
    return 0

$ docsig t.py
t.py:2 in f
    SIG003: unknown module comment option for disable ' SIG503' (unknown-module-directive-option)
    SIG503: return missing from docstring (return-missing)
$ echo $?
1

Expected: exit 0 — both rules disabled.