Catch signal handler change error (#1147)

The interruptible signal handle was incompatible with dask, it would trigger the following error:
```
Exception: "ValueError('signal only works in main thread of the main interpreter')"
```

This PR fixes the problem by catching the error and keeping the original signal handler in that case.

Authors:
  - Tamas Bela Feher (https://github.com/tfeher)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #1147

python/pylibraft/pylibraft/common/interruptible.pyx

@@ -54,11 +54,17 @@ def cuda_interruptible():
         with nogil:
             dereference(token).cancel()
 
-    oldhr = signal.signal(signal.SIGINT, newhr)
+    try:
+        oldhr = signal.signal(signal.SIGINT, newhr)
+    except ValueError:
+        # the signal creation would fail if this is not the main thread
+        # That's fine! The feature is disabled.
+        oldhr = None
     try:
         yield
     finally:
-        signal.signal(signal.SIGINT, oldhr)
+        if oldhr is not None:
+            signal.signal(signal.SIGINT, oldhr)
 
 
 def synchronize(stream: Stream):