Recent python 3 versions provide the built-in json module which meet
generic requirements. Remove explicit dependency on simplejson and use
the built-in module by default, to reduce external dependencies.
Also fix the missing requests library in requirements.
Depends-on: https://review.opendev.org/920001
Change-Id: Id8d4cec6fe709d1b647281ec4ff76b5104ec88f9