Make Gridliner deprecation warning show up

[rcomer]

Rationale

Follow up to #2330. The deprecation warning is currently not showing for normal use. I thought I had checked but perhaps I only looked in test output. I've now set the stacklevel to 3 for when Gridliner is created via ax.gridlines, and to 2 for the possible (but unlikely?) cases where a Gridliner is created directly.

I have checked locally that this now gives the desired results

In [1]: import matplotlib.pyplot as plt
   ...: import cartopy.crs as ccrs
   ...: fig = plt.figure()
   ...: ax = fig.add_subplot(projection=ccrs.PlateCarree())
   ...: ax.coastlines()
Out[1]: <cartopy.mpl.feature_artist.FeatureArtist at 0x756842d2d1c0>

In [2]: ax.gridlines(draw_labels=True, auto_update=True)
<ipython-input-2-499091cee3ba>:1: DeprecationWarning: The auto_update parameter was deprecated at Cartopy 0.23.  In future the gridlines and labels will always be updated.
  ax.gridlines(draw_labels=True, auto_update=True)
Out[2]: <cartopy.mpl.gridliner.Gridliner at 0x75688bcc5790>

In [3]: from cartopy.mpl.gridliner import Gridliner

In [4]: test = Gridliner(ax, ccrs.PlateCarree(), auto_update=True)
<ipython-input-4-422209944b61>:1: DeprecationWarning: The auto_update parameter was deprecated at Cartopy 0.23.  In future the gridlines and labels will always be updated.
  test = Gridliner(ax, ccrs.PlateCarree(), auto_update=True)

Implications

[rcomer]

#2394 suggests we have a bad interaction between the label placement logic and bbox_inches='tight'. I have added a comment that that should be addressed before we expire the deprecation.

[greglucas]

I'm not sure we should be using the private method here. The documentation says it may not be available in all reference Python implementations.
https://docs.python.org/3/library/sys.html#sys._getframemodulename

I think you could possibly use the inspect module instead, but it looks like in other code locations we've just explicitly set the stacklevel to 2, is that enough to handle this and not special case different stack levels?

[rcomer]

Thanks - I didn't know inspect had something for that. Now updated. If we leave the stacklevel at 2 then the warning won't show for users who are calling ax.gridlines. I was semi-following Matplotlib's warn_external which uses sys._getframe.