test: let the interop suites find a Python that actually has h5py

Every Python interop suite had stopped running on this machine: the h5py
writer round-trips, the facade suite, netCDF4 and the reference files.
`python3` is 3.14, nothing on the box has h5py, and PEP 668 refuses to
install it into a system interpreter at all — so the availability probes
all returned false and each suite skipped without failing.

A silent skip here is exactly how the v5 compound-datatype bug reached a
release, so the probes now read `CLAWHDF5_PYTHON` and `ci-test.sh` picks
up `.venv/bin/python` on its own. The detection sits at the top of the
script rather than beside the interop step, because the non-ignored
suites run in the earlier `cargo test` step and would otherwise still
miss it. `CLAWHDF5_REQUIRE_INTEROP=1` continues to turn a skip into a
failure.

Verified against a venv with h5py 3.16 / HDF5 2.0.0: 94 interop tests
across the four suites, all passing.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

crates/clawhdf5-format/tests/writer_h5py_tests.rs

@@ -4,9 +4,18 @@
 //! (and vice versa). They require python3 + h5py to be installed.
 
 use clawhdf5_format::file_writer::{AttrValue, CompoundTypeBuilder, EnumTypeBuilder, FileWriter};
+/// The Python interpreter to drive interop checks with.
+///
+/// `CLAWHDF5_PYTHON` lets these run against a virtualenv holding h5py, which
+/// on a PEP 668 "externally managed" system is the only place it can be
+/// installed. Without it the suite silently skips, and a silent skip here is
+/// how a datatype bug once reached a release.
+fn python() -> String {
+    std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
+}
 
 fn h5py_available() -> bool {
-    std::process::Command::new("python3")
+    std::process::Command::new(python())
         .args(["-c", "import h5py"])
         .output()
         .map(|o| o.status.success())
@@ -17,10 +26,10 @@ fn h5py_read(_path: &std::path::Path, script: &str) -> String {
     if !h5py_available() {
         panic!("h5py not installed — skipping interop test");
     }
-    let o = std::process::Command::new("python3")
+    let o = std::process::Command::new(python())
         .args(["-c", script])
         .output()
-        .expect("python3");
+        .expect("python interpreter");
     if !o.status.success() {
         panic!("h5py: {}", String::from_utf8_lossy(&o.stderr));
     }