Adapted the code of the non-daemonic pool to recent python versions

fixes #9739

The non-daemonic pool relies heavily on implementation details of the
multiprocessing package. In this commit we provide an implementation
that fits recent python versions.

lib/spack/spack/util/web.py

@@ -60,18 +60,30 @@ def handle_starttag(self, tag, attrs):
 
 class NonDaemonProcess(multiprocessing.Process):
     """Process tha allows sub-processes, so pools can have sub-pools."""
-    def _get_daemon(self):
+    @property
+    def daemon(self):
         return False
 
-    def _set_daemon(self, value):
+    @daemon.setter
+    def daemon(self, value):
         pass
 
-    daemon = property(_get_daemon, _set_daemon)
 
+if sys.version_info[0] < 3:
+    class NonDaemonPool(multiprocessing.pool.Pool):
+        """Pool that uses non-daemon processes"""
+        Process = NonDaemonProcess
+else:
 
-class NonDaemonPool(multiprocessing.pool.Pool):
-    """Pool that uses non-daemon processes"""
-    Process = NonDaemonProcess
+    class NonDaemonContext(type(multiprocessing.get_context())):
+        Process = NonDaemonProcess
+
+    class NonDaemonPool(multiprocessing.pool.Pool):
+        """Pool that uses non-daemon processes"""
+
+        def __init__(self, *args, **kwargs):
+            kwargs['context'] = NonDaemonContext()
+            super(NonDaemonPool, self).__init__(*args, **kwargs)
 
 
 def _spider(url, visited, root, depth, max_depth, raise_on_error):
@@ -310,7 +322,7 @@ def find_versions_of_archive(archive_urls, list_url=None, list_depth=0):
         #   .sha256
         #   .sig
         # However, SourceForge downloads still need to end in '/download'.
-        url_regex += '(\/download)?$'
+        url_regex += r'(\/download)?$'
 
         regexes.append(url_regex)