Trac #31306: sage.repl: Replace use of SAGE_EXTCODE by importlib.resources

We eliminate direct reading of files from the package directories of
sagelib by using `importlib.resources` (available since python 3.7)
- `git grep 'SAGE_EXTCODE' src/sage`
- `git grep '__file__' src/sage`

This will help make sagelib `zip_safe` (https://setuptools.readthedocs.i
o/en/latest/userguide/miscellaneous.html#setting-the-zip-safe-flag)

In this ticket, we take care of `sage.repl`, moving data needed by its
doctests from `SAGE_EXTCODE` (= `src/sage/ext_data`) into the package
directory. (Thus, we avoid a dependency on support for resources in
namespace packages brought by Python 3.10(?) or the backport package
`importlib-resources`, see
python/importlib_resources#196)

Follow-up tickets will deal with other parts of the library. In the end,
the directory `src/sage/ext_data` will be eliminated.

References:
- https://docs.python.org/3/library/importlib.html#module-
importlib.resources
- https://importlib-resources.readthedocs.io/en/latest/migration.html
- https://importlib-resources.readthedocs.io/en/latest/using.html

URL: https://trac.sagemath.org/31306
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

src/sage/repl/display/formatter.py

@@ -142,16 +142,16 @@ def format(self, obj, include=None, exclude=None):
         TESTS::
 
             sage: import os
-            sage: from sage.env import SAGE_EXTCODE
-            sage: example_png = os.path.join(SAGE_EXTCODE, 'doctest', 'rich_output', 'example.png')
+            sage: import importlib.resources
             sage: from sage.repl.rich_output.backend_ipython import BackendIPython
             sage: backend = BackendIPython()
             sage: shell = get_test_shell()
             sage: backend.install(shell=shell)
             sage: shell.run_cell('get_ipython().display_formatter')
             <sage.repl.display.formatter.SageDisplayFormatter object at 0x...>
             sage: shell.run_cell('from IPython.display import Image')
-            sage: shell.run_cell('ipython_image = Image("{0}")'.format(example_png))
+            sage: with importlib.resources.path(sage.repl.rich_output, 'example.png') as example_png:
+            ....:     shell.run_cell('ipython_image = Image("{0}")'.format(example_png))
             sage: shell.run_cell('ipython_image')
             <IPython.core.display.Image object>
             sage: shell.run_cell('get_ipython().display_formatter.format(ipython_image)')