Fix for issue 89, now works with PyObjC 3.0+

[igorbogdanov]

Due to the change of rules for mapping python into obj-c in 3.0+ versions of PyObjC this command line tool could not be used with 3.0+ PyObjC, generating Error described in in Issue 89. The issue is now fixed, but not sure if it is backwards compatible so suggest having this fix as a separate branch.

[paulhammond]

Thanks for submitting this fix. I tried it on my Mac (Catalina, 10.15.3) which has pyobjc 2.5.1 installed, and I get this error:

Traceback (most recent call last):
  File "./webkit2png2", line 78, in <module>
    class WebkitLoad (Foundation.NSObject, objc.protocolNamed('WebFrameLoadDelegate')):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/PyObjC/objc/_protocols.py", line 33, in protocolNamed
    raise ProtocolError("protocol %r does not exist" % (name,), name)
objc.ProtocolError: ("protocol u'WebFrameLoadDelegate' does not exist", u'WebFrameLoadDelegate')

My goal with webkit2png is to have it work out of the box on a Mac, without having to install extra software, so a version that doesn't work with 2.5.1 isn't an option. I don't know if there's a backwards compatible way to make this work for both versions of pyobjc?