Fix compatibility with python3.9 (#43)

Author: rmcgibbo

imageio_ffmpeg/_parsing.py

@@ -43,7 +43,7 @@ def get_text(self, timeout=0):
         # Wait?
         if timeout > 0:
             etime = time.time() + timeout
-            while self.isAlive() and time.time() < etime:  # pragma: no cover
+            while self.is_alive() and time.time() < etime:  # pragma: no cover
                 time.sleep(0.01)
         # Return str
         lines = b"\n".join(self._lines)