bpo-46820: Fix a SyntaxError in a numeric literal followed by "not in" (GH-31479) (GH-31493)

Fix parsing a numeric literal immediately (without spaces) followed by
"not in" keywords, like in "1not in x". Now the parser only emits
a warning, not a syntax error.
(cherry picked from commit 090e5c4b946b28f50fce445916c5d3ec45c8f45f)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>