Bump 1.7.3 for setup.py tweak
Merge branch '1.7' into 1.8 & cut 1.8.3

Conflicts:
	fabric/version.py
Permit Paramiko >=1.13 under Python 2.6+

Re #1105
Changelog re #1105, fixes #1105
Merge branch '1.7' into 1.8

Conflicts:
	docs/changelog.rst