[pp:exec] use 'False' as default for 'start_new_session' to avoid TypeError (#7899)

in Python <= 3.11

* Use False as default instead of None to avoid TypeError
* Update postprocessor tests

gallery_dl/postprocessor/exec.py

@@ -35,7 +35,7 @@ class ExecPP(PostProcessor):
             if options.get("async", False):
                 self._exec = self._popen
 
-        self.session = None
+        self.session = False
         self.creationflags = 0
         if options.get("session"):
             if util.WINDOWS:

test/test_postprocessor.py

@@ -211,7 +211,7 @@ class ExecTest(BasePostprocessorTest):
                 self.pathfmt.filename),
             shell=True,
             creationflags=0,
-            start_new_session=None,
+            start_new_session=False,
         )
         i.wait.assert_called_once_with()
 
@@ -235,7 +235,7 @@ class ExecTest(BasePostprocessorTest):
             ],
             shell=False,
             creationflags=0,
-            start_new_session=None,
+            start_new_session=False,
         )
 
     def test_command_many(self):
@@ -260,7 +260,7 @@ class ExecTest(BasePostprocessorTest):
                     self.pathfmt.filename),
                 shell=True,
                 creationflags=0,
-                start_new_session=None,
+                start_new_session=False,
             ),
             call(
                 [
@@ -270,7 +270,7 @@ class ExecTest(BasePostprocessorTest):
                 ],
                 shell=False,
                 creationflags=0,
-                start_new_session=None,
+                start_new_session=False,
             ),
         ])