Attempting to fix psutil dep due to https://github.com/giampaolo/psutil/issues/1659

setup.py

@@ -279,14 +279,19 @@ EXTRA_EVENTS = [
     'zope.event',
     'zope.interface',
 ]
-# Fails to build on PyPy on Windows.
-# TODO: Is that still the case?
-EXTRA_PSUTIL_DEP = 'psutil >= 5.6.1 ; platform_python_implementation == "CPython" or sys_platform != "win32"'
 
-EXTRA_MONITOR = [
-    EXTRA_PSUTIL_DEP,
+EXTRA_PSUTIL_DEPS = [
+    # psutil fails  to build on PyPy on Windows. (TODO: Is that still the case?)
+    # Versions of PyPy2 prior to 7.4 (maybe?) are incompatible with
+    # psutil >= 5.6.4.
+    # https://github.com/giampaolo/psutil/issues/1659
+    'psutil >= 5.6.1 ; platform_python_implementation == "CPython" or python_version!="2.7"',
+    'psutil == 5.6.3 ; platform_python_implementation=="PyPy" and python_version=="2.7" and sys_platform != "win32"',
 ]
 
+EXTRA_MONITOR = [
+] + EXTRA_PSUTIL_DEPS
+
 EXTRA_RECOMMENDED = [
     # We need this at runtime to use the libev-CFFI and libuv backends
     CFFI_DEP,