Use argparse's choices= to display OUTPUT_CHOICES

[wyattscarpenter]

This allows the user to see all of the relevant providable values. If a metavar is provided it cloaks the choices, so we remove the metavar. I also document why this can't be done for --enable-incomplete-feature.

There are no tests. I manually examined the generated results and found them satisfactory. They do the obvious.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

AutoSplit (https://github.com/Toufool/AutoSplit)
-   File "/tmp/mypy_primer/mypy_old/venv/bin/mypy", line 7, in <module>
+   File "/tmp/mypy_primer/mypy_new/venv/bin/mypy", line 7, in <module>
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/__main__.py", line 15, in console_entry
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/__main__.py", line 15, in console_entry
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/main.py", line 127, in main
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/main.py", line 127, in main
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/main.py", line 211, in run_build
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/main.py", line 211, in run_build
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/build.py", line 196, in build
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/build.py", line 196, in build
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/build.py", line 272, in _build
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/build.py", line 272, in _build
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/build.py", line 2946, in dispatch
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/build.py", line 2946, in dispatch
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/build.py", line 3346, in process_graph
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/build.py", line 3346, in process_graph
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/build.py", line 3475, in process_stale_scc
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/build.py", line 3475, in process_stale_scc
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/build.py", line 2493, in write_cache
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/build.py", line 2493, in write_cache
-   File "/tmp/mypy_primer/mypy_old/venv/lib/python3.13/site-packages/mypy/cache.py", line 28, in __init__
+   File "/tmp/mypy_primer/mypy_new/venv/lib/python3.13/site-packages/mypy/cache.py", line 28, in __init__

[wyattscarpenter]

(Informational:) an alternate form I considered for this code was

        "-O",
        "--output",
        metavar="FORMAT",
        # The metavar overrides the default of displaying the choices,
        # so we have to explicitly display them.
        help=f"Set a custom output format (choices: {set(OUTPUT_CHOICES.keys())})",
        choices=OUTPUT_CHOICES,

However, I ultimately decided that just letting it do its default thing was better.