issue#1009

form -> from

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setuptools doesn't like it.

- hddtemp is not installable using pip, remove it.
- conditionalize scandir dependency
- add py3sensors

Note: pip >= 6.0 is required.

- scandir package is only needed for Python < 3.5.
- the name of extras are now lower case (easier to type/read).

Note: setuptools >= 18.0 is required.

Just to be clear.

Some cleaning while I'm here.

That's way better than checking for the OS; psutil may implement it for
new platforms in the future.

As of version 4.3.1, psutil's Process.as_dict() raises ValueError if an
attribute name is invalid.

See e2912943.