Add weakrefmethod to requirements for python2.7

This library provides the equivalent functionality of weakrefmethod
in python3. This will be used by Neutron. This patch doesn't update
upper-constraints.txt because we implicitly depend on this library
so it's already present.

- Is the library actively maintained?

Not really, but it's a one function library that accomplishes its goal
so there isn't much to maintain.

- Is the library good code?

Yes, it's pretty easy to follow for the complex thing it achieves
(weak references to bound methods).

- Is the library python 3 compatible?

No, but the purpose of this library is to provide a python3 function
so we can use it in python2.

- Is the library license compatible?

Yes. It's PSF.

- Is the library already packaged in the distros we target (Ubuntu
  latest / Fedora latest)?

I assume so, we already depend on it via the notifier library.

- Is the function of this library already covered by other libraries
  in ``global-requirements.txt``?

No

- Is the library required for OpenStack project or related dev or
  infrastructure setup? (Answer to this should be Yes, of course)
  Which?

Yes, Neutron for a callback handling system. See
I7576851a44abd14cbc337a3d3e28690c7316ec81

Change-Id: I8e6173342bd2ab4f9f31b3aaa1452e838770fa06

global-requirements.txt

@@ -186,6 +186,7 @@ pydotplus>=2.0.2   # MIT License
 crc16>=0.1.1  # LGPLv3+
 pyzabbix>=0.7.4  # LGPL
 statsd>=3.2.1  # MIT
+weakrefmethod>=1.0.2;python_version=='2.7'  # PSF
 
 # NOTE(flaper87): This added 2 packages to `integration.sh`
 # uuid-dev and swig