fix: Don't run so many OpenBlas threads.

Importing all of numpy loads OpenBlas which spawns many worker threads
behind the scenes.  Increasing the number of threads didn't seem to
work. I got up to 30 before deciding that it would be better for the
test to just limit the number of OpenBLAS threads.

The addition of the following code seems to resolve the issue:

```
import os
os.environ['OPENBLAS_NUM_THREADS'] = '1'
```

Reference: https://stackoverflow.com/questions/52026652/openblas-blas-thread-init-pthread-create-resource-temporarily-unavailable

codejail/tests/test_safe_exec.py

@@ -101,6 +101,8 @@ def test_importing_lots_of_crap(self):
         set_limit('REALTIME', 10)
         globs = {}
         self.safe_exec(textwrap.dedent("""\
+            import os
+            os.environ['OPENBLAS_NUM_THREADS'] = '1'
             from numpy import *
             a = 1723
             """), globs)