runtime(python): Update syntax file, fix f-string float highlighting

Fix matching of floats at the beginning of an f-string replacement
field, immediately after the opening brace.

The existing pattern, using `\zs`, cannot consume the already matched
`{` so use a lookbehind instead.

See comment: https://github.com/vim/vim/pull/17962#issuecomment-3201550443

closes: #18220

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/python_strings_bytes.py

@@ -108,6 +108,7 @@ test = f"Number is {
     1 +
     2 +
     3}"
+test = f"Float is {1.23}"
 test = f"abc{a # This is a comment }
     + 1}"
 test = f"def{a # So is this :