sysutils/py-plumbum: update to 1.6.3

• Python 3.6 is now supported, critical bug fixed (#302)
• Commands: Better handling of return codes for pipelines (#288)
• Paths: Return split support (regression) (#286) - also supports dummy args for better `str` compatibility
• Paths: Added support for Python 3.6 path protocol
• Paths: Support Python's `in` syntax
• CLI: Added Config parser (provisional) (#304)
• Color: image plots with python -m plumbum.cli.image (#304)
• SSH: No longer hangs on failure (#306)
• Test improvements, especially on non-linux systems