Allow literal string argument that starts with a "#".

Without this change, when running setup.py extract_messages, we would get
error:

    TokenError: ('EOF in multi-line statement', (2, 0))

Looking at the unit test, it seems like a literal string argument in the form
of "_('xxx')" is supposed to be picked up by Babel. However, that contradicts
with the documentation, which states:

> When using tags, the values of the arguments are taken as literal strings by
> default. To embed Python expressions as arguments, use the embedded
> expression format

So, as a side effect of this commit, the convenience of "_('xxx')" has been
removed. Probably can add it back if necessary for backward compatibility.