Pyflyby should understand structural pattern matching (PyInf#11543)

[dshivashankar1994]

Currently pyflyby does not understand structural pattern matching. In the following example, pyflyby erroneously suggests an unneeded import:

from typing_extensions import assert_never

my_var: int | str = 5

match my_var:
    case str():
        print("we have a str")
    case int():
        print("we have an int")
    case json:  # try to assert that my_var is never a json object
        assert_never(json)

~ tidy-imports pattern_match.py
[PYFLYBY] /u/zeng/repos/scratch/pattern_match.py: added 'import json'
--- /u/zeng/repos/scratch/pattern_match.py      2023-11-29 12:32:59.404921000 +0000
+++ /tmp/tmps3uv669v    2023-11-29 12:33:06.071715619 +0000
@@ -1,4 +1,5 @@
-from typing_extensions import assert_never
+import json
+from   typing_extensions        import assert_never
 
 my_var: int | str = 5
 

Replace /u/zeng/repos/scratch/pattern_match.py? [y/N] 

case json: is a catch-all that will match any value and assign the name json to it, though that does not detract from this being a pyflyby bug.