Changed ImageMath.ops to be static

[radarhere]

Resolves #7717

[hugovk]

Looks good, thanks.

I don't expect we'll add new imagemath_* functions often, but if we do, is it worth adding a test to make sure we remember to add it to the dict?

[homm]

If some function is not listed in the dict, it wouldn't work in eval, right? So any test will caught this

[hugovk]

Ah, that's right, temporarily commenting one one:

diff --git a/src/PIL/ImageMath.py b/src/PIL/ImageMath.py
index a7652f237..47e552d3a 100644
--- a/src/PIL/ImageMath.py
+++ b/src/PIL/ImageMath.py
@@ -231,7 +231,7 @@ ops = {
     "notequal": imagemath_notequal,
     "min": imagemath_min,
     "max": imagemath_max,
-    "convert": imagemath_convert,
+    # "convert": imagemath_convert,
 }

Fails like this:

Tests/test_imagemath.py ........F..................                                                                              [100%]

=============================================================== FAILURES ===============================================================
_____________________________________________________________ test_convert _____________________________________________________________

    def test_convert():
>       assert pixel(ImageMath.eval("convert(A+B, 'L')", images)) == "L 3"

Tests/test_imagemath.py:84:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
src/PIL/ImageMath.py:276: in eval
    scan(compiled_code)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

code = <code object <module> at 0x107002f70, file "<string>", line 1>

    def scan(code: CodeType) -> None:
        for const in code.co_consts:
            if type(const) is type(compiled_code):
                scan(const)

        for name in code.co_names:
            if name not in args and name != "abs":
                msg = f"'{name}' not allowed"
>               raise ValueError(msg)
E               ValueError: 'convert' not allowed

src/PIL/ImageMath.py:274: ValueError
======================================================= short test summary info ========================================================
FAILED Tests/test_imagemath.py::test_convert - ValueError: 'convert' not allowed
===================================================== 1 failed, 26 passed in 1.80s =====================================================