runtime: avoid assumption that all objects provide __call__

[berrange]

Objects which are backed by native extensions do not provide
a __call__ attribute, but are none the less callable if the
native extension provides a tp_call implementation.

The jinja2.runtime.Context.call method unconditionally
access the __call__ attribute causing an exception to be
raised if the object was a native extension method.

A demo of the problem can be seen using PyGObject:

  $ cat demo.py
  #!/usr/bin/python

  from gi.repository import Gio
  from jinja2 import Environment

  f = Gio.File.new_for_path("/some/file.txt")
  print f.get_uri()

  t = Environment().from_string("""{{f.get_uri()}}""")
  print t.render(f=f)

Which when run results in

 $ ./demo.py
 file:///some/file.txt
 Traceback (most recent call last):
   File "./demo.py", line 10, in <module>
     print t.render(f=f)
   File "/usr/lib/python2.7/site-packages/jinja2/environment.py", line 969, in render
     return self.environment.handle_exception(exc_info, True)
   File "/usr/lib/python2.7/site-packages/jinja2/environment.py", line 742, in handle_exception
     reraise(exc_type, exc_value, tb)
   File "<template>", line 1, in top-level template code
     AttributeError: 'gi.FunctionInfo' object has no attribute '**call**'

After this patch it produces

 $ ./demo.py
 file:///some/file.txt
 file:///some/file.txt

Signed-off-by: Daniel P. Berrange berrange@redhat.com

[ThiefMaster]

just a small heads-up: you can use fenced code blocks (triple backticks before and after) to format something as code on github. right now your demo code in the PR's description is not very readable

[berrange]

@ThiefMaster all the text in the PR was just automatically included by github from the commit message berrange@91255f8

[davidism]

@mitsuhiko checking if the callable object is a FunctionType or MethodType seems unnecessary. Removing it doesn't break any tests and allows callable C objects to mark themselves as context functions too by setting a class attribute instead of a function attribute.

Is there a reason not to remove the isinstance(obj, _context_function_types) check?