Limit workers in ProcessPoolExecutor

As a mitigation for #1730, this commit limits the number of workers in
the ProcessPoolExecutor to 1 (default is the number of CPUs). On PyPy,
having a higher number of available workers dramatically increases the
number of concurrent processes, leading to some resource exhaustion
issues.

This does not address the root issue, but should improve the situation
until the root issue is addressed.

setuptools/tests/test_build_meta.py

@@ -28,7 +28,7 @@ class BuildBackend(BuildBackendBase):
 
     def __init__(self, *args, **kwargs):
         super(BuildBackend, self).__init__(*args, **kwargs)
-        self.pool = futures.ProcessPoolExecutor()
+        self.pool = futures.ProcessPoolExecutor(max_workers=1)
 
     def __getattr__(self, name):
         """Handles aribrary function invocations on the build backend."""