Replace unittest by pytest for our test runner

We've been having multiple discussions regarding the test suite of rez over in Slack. The first step to improve the situation is to switch to pytest for running our tests. It is important to note that for now, `rez-selftest` will stay and its behavior won't change.

The main benefit of using pytest simply to run the tests will be:
* Way cleaner and readable test output (we will see the rez output only if a test fails).
* Better error reporting

Later on (another issue will need to be created), we will be able to rewrite our tests to completely use pytest which will give us all the good benefits of pytest (easy coverage even in subprocesses, fixtires, marks, etc).