cython-mode.el: add missing grouping to keyword regexps

It appears that the improvement in Emacs highlighting of cython builtins (#279) broke things, which I didn't notice when testing, probably because I forgot to recompile the module after the last changes. I've finally got to update my configuration on a different host and the fact it didn't work became obvious.

Here's a fix.

If anyone fancies testing it to avoid falling for the same mistake, here's the command to do that (provided you have Emacs installed):
```bash
$ emacs -Q -l Tools/cython-mode.el Cython/Utility/CppConvert.pyx
```

It also looks like that some of the (rather) new keywords, such as `fused` or `cppclass` are not supported, should I add them in this PR or in a new one?