ci: express "no x86 for pre-3.11 Python" as a single matrix policy

The previous commit listed three explicit (os, python, architecture)
triples to exclude. Since x86 is already excluded on ubuntu-latest and
macos-latest by their own (os, architecture) rules, the os qualifier
on these entries is redundant.

Replace the triples with (python, architecture) pairs so the policy
reads as "no x86 testing for Python below 3.11" — which is the actual
rule we care about. cp311-abi3-win32 wheels from `cryptography` cover
3.11+, so those cells remain in the matrix. No change in effective
coverage.

Author: josecorella

.github/workflows/ci_tests.yaml

@@ -73,17 +73,13 @@ jobs:
           # (Rust/PyO3). That build fails on the hosted windows-latest
           # runner because only the x86_64 Rust target is installed while
           # the interpreter reports platform `win32` (i686), so maturin
-          # refuses to use it. The job is therefore unbuildable on
-          # GitHub-hosted runners until a host-side toolchain workaround
-          # exists; drop these combinations from the matrix.
-          - os: windows-latest
-            python: 3.8
+          # refuses to use it. Until a host-side toolchain workaround
+          # exists, omit pre-3.11 Python from x86 testing entirely.
+          - python: 3.8
             architecture: x86
-          - os: windows-latest
-            python: 3.9
+          - python: 3.9
             architecture: x86
-          - os: windows-latest
-            python: "3.10"
+          - python: "3.10"
             architecture: x86
 
           # MPL is not supported on <3.11