IOTBTOOL-400: Fix mbed add in python 3

mbed add command would fail if trying to pull a mercurial repo since
mercurial is not in the requirements list. Added it (now supports py3)

Author: mark-edgeworth

setup.py

@@ -47,5 +47,6 @@
     install_requires=[
         "pyserial>=3.0,<4.0",
         "mbed-os-tools>=0.0.9,<0.1.0",
+        "mercurial>=5.2"
     ]
 )