python: make --enable-pylint on my default

[chu11]

When submitting #1748, I was surprised to hit pylint errors and wasn't aware of the --enable-pylint configure option at first. If python is a first class part of flux now (i.e. #1772), lets default to run pylint as long as it is available. Users can --disable-pylint at their discretion still.

[grondo]

We've gone awhile without this one and I'm not sure it would be good to require pylint by default so closing.