fix(ci): unpin mypy version in mypy workflow (partial cherry-pick of #3678)

The pinned `requirements: 1.6.0` and `python_version: 3.9` no longer work
with the current `jpetrucciani/mypy-check@master` action, which now
defaults to `mypy==2.1.0` (Requires-Python >=3.10). This drops the pins
so the action picks its defaults — same as on `main` and `release/2.0`.

Workflow-only hunk from commit 782ae91 ("fix(uv): drop powerpc64
support to fix latest version downloads" (#3678)); the uv/MODULE.bazel
parts of that commit are not applicable to `release/1.9`.

(cherry picked from commit 782ae91)

Author: aignas

.github/workflows/mypy.yaml

@@ -21,11 +21,7 @@ jobs:
       - uses: actions/checkout@v6
       - uses: jpetrucciani/mypy-check@master
         with:
-          requirements: 1.6.0
-          python_version: 3.9
           path: 'python/runfiles'
       - uses: jpetrucciani/mypy-check@master
         with:
-          requirements: 1.6.0
-          python_version: 3.9
           path: 'tests/runfiles'