Pin traits>=6.2.0

[rahulporuri]

Now that we are in the middle of moving from on_trait_change to observe, we need to enforce traits>=6.2.0 in our dependencies. This is a release blocker for the 5.1.0 enable release (ref #719 )

[jwiggins]

What needs to be done here? A change to __requires__?

[rahulporuri]

A change to requires?

yep. that's all.