Add six to requirements

[rahulporuri]

six is now a runtime requirement because we removed the use of 2to3 in the package.
~ numpy and pandas have been runtime requirements of a few submodules of apptools which were never reported.~

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@@            Coverage Diff            @@
##             master     #101   +/-   ##
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Impacted Files	Coverage Δ
apptools/__init__.py	60% <ø> (ø)

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[corranwebster]

Strong recommendation that Pandas is not a runtime requirement but an optional requirement instead (not sure what exactly requires it, but we don't want people having to install by default).

[rahulporuri]

Strong recommendation that Pandas is not a runtime requirement but an optional requirement instead (not sure what exactly requires it, but we don't want people having to install by default).

yep yep. exactly @mdickinson 's concerns as well.
updating.

[mdickinson]

LGTM