* Cleanup unused imports (F401) of mostly standard Python modules,
  or some internal but unlikely referenced modules
* Where internal imports are potentially used, mark with noqa
* Avoid redefinition of imports (F811)

Inheriting from object was necessary for Python 2 compatibility to use
new-style classes. In Python 3, this is unnecessary as there are no
old-style classes.

Dropping the object is more idiomatic Python.

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

See also: #13880

* Use list comprehension

* More list comprehension migration

* Revert key copying in dict

* A few more fixes

* More reverts

* Use dict comprehension

* Fix dict comprehension

* Address review comments

* More review comments

* Fix for empty unpacking of zip(*

* Revert zip(* unpacking altogether

* Fix dict copying

* More simplifications

In Python 3.6 a number of escape sequences that were previously accepted
-- for instance "\(" that was translated to "\\(" -- are deprecated. To
retain the previous behavior either raw strings must be used or the
backslash must be properly escaped itself.

Closes #7572 inability to install in virtualenvs with percent in their path.