wcsaxes get_transform errors when sliced WCS given

[DanRyanIrish]

Description

wcsaxes.transforms.get_transform does not recognise sliced WCS objects. When one is entered, a TypeError is returned saying TypeError: output_system should be a WCS instance, string, or a coordinate frame instance

Expected behavior

Actual behavior

Steps to Reproduce

1. [First Step]
2. [Second Step]
3. [and so on...]

# Put your Python code snippet here.
# Let wcs1 and wcs2 be two FITS WCS objects, describing arrays data1 and data2.
# data1/wcs1 is an image, while data2/wcs2 is an image cube.  So slice data2/wcs2 to overplot.
>>> data3 = data2[0]
>>> wcs3 = wcs2[0]

>>> import matplotlib
>>> ax = plt.subplot(projection=wcs1)
>>> ax.imshow(data1)
>>> ax.contour(data3, transform=ax.get_transform(wcs3), colors='red')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_26357/1992842180.py in <module>
      1 ax = plt.subplot(projection=wcs1)
      2 ax.imshow(data1)
----> 3 ax.contour(data3, transform=ax.get_transform(wcs3), colors='red')

~/miniconda3/envs/foxsi2/lib/python3.9/site-packages/astropy/visualization/wcsaxes/core.py in get_transform(self, frame)
    564                 * :class:`~astropy.coordinates.BaseCoordinateFrame` instance.
    565         """
--> 566         return self._get_transform_no_transdata(frame).inverted() + self.transData
    567 
    568     def _get_transform_no_transdata(self, frame):

~/miniconda3/envs/foxsi2/lib/python3.9/site-packages/astropy/visualization/wcsaxes/core.py in _get_transform_no_transdata(self, frame)
    603             else:
    604 
--> 605                 coordinate_transform = CoordinateTransform(self._transform_pixel2world.frame_out, frame)
    606 
    607                 if coordinate_transform.same_frames:

~/miniconda3/envs/foxsi2/lib/python3.9/site-packages/astropy/visualization/wcsaxes/transforms.py in __init__(self, input_system, output_system)
     87             self.output_system = self._output_system_name
     88         else:
---> 89             raise TypeError("output_system should be a WCS instance, string, or a coordinate frame instance")
     90 
     91         if self.output_system == self.input_system:

TypeError: output_system should be a WCS instance, string, or a coordinate frame instance

System Details

Linux-5.8.0-59-generic-x86_64-with-glibc2.31
Python 3.9.6 | packaged by conda-forge | (default, Jul 11 2021, 03:39:48)
[GCC 9.3.0]
Numpy 1.21.1
astropy 4.2.1 (Same issue with 4.3rc1)
Scipy 1.7.0
Matplotlib 3.4.2