FIX/API: Do not reset rcParams['backend'] in rc_context

The motivating issue is that if the backend was the auto backend sentinel and
the backend was first fully resolved in an `rc_context` block, then we would
reset back to the sentinel an exit which would lead to strange behavior with
figures being erroneously closed.

This special cases the 'backend' key to not be reset on `__exit__`

Closes matplotlib#23298

Author: tacaswell

doc/api/next_api_changes/behavior/23299-TAC.rst

@@ -0,0 +1,9 @@
+mpl.rc_context no longer resests the `'backend'` key
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you want to rest :rc:`backend` after the context, you must use
+`matplotlib.use` to change the backend.  Previously the key would be reset, but
+it would have any of the expected side-effects because internally
+`matplotlib.rc_context` side-steps the custom logic in the
+`~matplotlib.RcParams` class (because we copied the values from ``rcParamms``
+so it does not need to be re-validated.

lib/matplotlib/__init__.py

@@ -1059,6 +1059,8 @@ def rc_context(rc=None, fname=None):
     """
     Return a context manager for temporarily changing rcParams.
 
+    The :rc:`backend` will not be reset by the context manager.
+
     Parameters
     ----------
     rc : dict
@@ -1087,7 +1089,8 @@ def rc_context(rc=None, fname=None):
              plt.plot(x, y)  # uses 'print.rc'
 
     """
-    orig = rcParams.copy()
+    orig = dict(rcParams.copy())
+    del orig['backend']
     try:
         if fname:
             rc_file(fname)

lib/matplotlib/tests/test_rcparams.py

@@ -496,6 +496,13 @@ def test_keymaps():
         assert isinstance(mpl.rcParams[k], list)
 
 
+def test_no_backend_reset_rccontext():
+    assert mpl.rcParams['backend'] != 'module://aardvark'
+    with mpl.rc_context():
+        mpl.rcParams['backend'] = 'module://aardvark'
+    assert mpl.rcParams['backend'] == 'module://aardvark'
+
+
 def test_rcparams_reset_after_fail():
     # There was previously a bug that meant that if rc_context failed and
     # raised an exception due to issues in the supplied rc parameters, the