Upgrade pyxb lib.

This is the recommended fix for the following issue:
AuthorizeNet#112

Author: wm3ndez

authorizenet/apicontractsv1.py

@@ -17,7 +17,7 @@
 _GenerationUID = pyxb.utils.utility.UniqueIdentifier('urn:uuid:d394854f-e894-11e8-964f-00155d846ce4')
 
 # Version of PyXB used to generate the bindings
-_PyXBVersion = '1.2.5'
+_PyXBVersion = '1.2.6'
 # Generated bindings are not compatible across PyXB versions
 if pyxb.__version__ != _PyXBVersion:
     raise pyxb.PyXBVersionError(_PyXBVersion)

setup.py

@@ -63,7 +63,7 @@
     # your project is installed. For an analysis of "install_requires" vs pip's
     # requirements files see:
     # https://packaging.python.org/en/latest/requirements.html
-    install_requires=['pyxb==1.2.5', 'lxml==4.*', 'requests==2.*'],
+    install_requires=['pyxb==1.2.6', 'lxml==4.*', 'requests==2.*'],
     #install_requires=['nose'],
     #install_requires=['nosetests'],
     #install_requires=['mock'],