Change callable check for `WampClientProtocol.subscribe`
Change callable check for `WampClientProtocol.subscribe`

Use a simpler check for callables that allows for any callable object.
(Previously did not allow callable classes.)

Jenselme/AutobahnPython,D3f0/AutobahnPython,oberstet/autobahn-python,wrapp/AutobahnPython,Geoion/AutobahnPython,dash-dash/AutobahnPython,dash-dash/AutobahnPython,iffy/AutobahnPython,crossbario/autobahn-python,iffy/AutobahnPython,jvdm/AutobahnPython,boonedox/AutobahnPython,leedm777/AutobahnPython,iffy/AutobahnPython,mcfletch/AutobahnPython,markope/AutobahnPython,crossbario/autobahn-python,oberstet/autobahn-python,bencharb/AutobahnPython,meejah/AutobahnPython,hzruandd/AutobahnPython,claws/AutobahnPython,crossbario/autobahn-python,inirudebwoy/AutobahnPython,dash-dash/AutobahnPython,D3f0/AutobahnPython,tomwire/AutobahnPython,RyanHope/AutobahnPython,nucular/AutobahnPython,mcfletch/AutobahnPython,claws/AutobahnPython,boonedox/AutobahnPython,tavendo/AutobahnPython,leedm777/AutobahnPython,dash-dash/AutobahnPython,ttimon7/AutobahnPython,schoonc/AutobahnPython,luhn/AutobahnPython,D3f0/AutobahnPython,oberstet/autobahn-python,wrapp/AutobahnPython,wrapp/AutobahnPython,iffy/AutobahnPython,leedm777/AutobahnPython,boonedox/AutobahnPython