gh-126807: pygettext: Do not attempt to extract messages from function definitions. (GH-126808)

Fixes a bug where pygettext would attempt
to extract a message from a code like this:

def _(x): pass

This is because pygettext only looks at one
token at a time and '_(x)' looks like a
function call.

However, since 'x' is not a string literal,
it would erroneously issue a warning.