fixup! tumblr_backup: Implement wget module

Avoid a deadlock in ThreadPool.cancel with a new function:
WaitOnMainThread.destroy, which prevents blocking in signal() when we're
trying to stop the thread

tumblr_backup.py

@@ -1414,6 +1414,7 @@ def wait(self):
 
     def cancel(self):
         self.abort.set()
+        no_internet.destroy()
         for i, t in enumerate(self.threads, start=1):
             log.status('Stopping threads {}{}\r'.format(' ' * i, '.' * (len(self.threads) - i)))
             t.join()

util.py

@@ -202,6 +202,16 @@ def check(self):
             self.flag = False
             self.cond.notify_all()
 
+    # Call on main thread to prevent threads from blocking in signal()
+    def destroy(self):
+        assert self.cond is not None
+        if self.flag is None:
+            return
+
+        with self.cond:
+            self.flag = None  # Cause all waiters to exit
+            self.cond.notify_all()
+
     def _do_wait(self):
         assert self.cond is not None
         if self.flag is None:
@@ -212,6 +222,7 @@ def _do_wait(self):
         except:
             with self.cond:
                 self.flag = None  # Waiting never completed
+                self.cond.notify_all()
             raise
 
     @staticmethod