psutil pinned to outdated version on PyPy

[jamadden]

• gevent version: 20.x; master
• Python version: PyPyX 7.3.1 (the only PyPy version we support)
• Operating System: All

Description:

(Speaking only about PyPy.)

On Windows, we don't depend on psutil at all because at one point it didn't build. It's not clear if that's still the case.

On other platforms, for PyPy2, we have psutil pinned to a specific older version (5.6.3) because of giampaolo/psutil#1659

But a quick test seems to show that PyPy2 7.3.1 and psutil 5.7.0 work OK together; the specific example in the PyPy bug report passes.

This is a problem for larger (application) installations that have a different pin on psutil.

Let's see how much we can relax this.