Upgrade pylint => 1.7.1

[troydai]

Update the pylint to version 1.7.1 and fixes the newly appeared issues. Notably:

:no-else-return (R1705): *Unnecessary "else" after "return"*
  else following a chain of ifs, all of them containing a return statement.

:len-as-condition (C1801): *Do not use `len(SEQUENCE)` as condition value*
  Used when Pylint detects incorrect use of len(sequence) inside conditions.
  implement __reversed__, nor __getitem__ and __len__

In addition, as a result of the upgrade, pylint is enabled in 3.6.

[troydai]

I would like to merge this change as soon as possible ti reduce the impacts from other pull requests since this impact a fairly large number of files.

[tjprescott]

Hoo... more pylint.

[yugangw-msft]

maybe filter_string or None?

[troydai]

Thanks for the quick turnaround!