fix: Increase the number of threads when importing everything.

Importing all of numpy loads OpenBlas which spawns many worker threads
behind the scenes.  We need to increase the number of threads that are
allowed when importing "lots of crap".

The other option would be to limit the number of threads that OpenBLAS
uses by setting the `OPENBLASH_NUM_THREADS` environment variable:

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

codejail/tests/test_safe_exec.py

@@ -99,6 +99,7 @@ def test_printing_stuff_when_you_shouldnt(self):
 
     def test_importing_lots_of_crap(self):
         set_limit('REALTIME', 10)
+        set_limit('NPROC', 30)
         globs = {}
         self.safe_exec(textwrap.dedent("""\
             from numpy import *