Fix cargo test linking: make extension-module a conditional feature

PyO3's extension-module feature skips linking against libpython, which
is correct for building the .so/.pyd extension but breaks cargo test.

Move extension-module behind a Cargo feature flag (enabled by default
so maturin builds are unaffected) and use --no-default-features in CI
for cargo test and clippy.

Author: nathanjmcdougall

.github/workflows/rust-test.yml

@@ -45,4 +45,4 @@ jobs:
           toolchain: ${{ matrix.rust }}
 
       - name: Run cargo test
-        run: cargo test
+        run: cargo test --no-default-features

.github/workflows/static-checks.yml

@@ -54,4 +54,4 @@ jobs:
               run: cargo fmt --check
 
             - name: Run clippy
-              run: cargo clippy -- -D warnings
+              run: cargo clippy --no-default-features -- -D warnings

Cargo.toml

@@ -10,12 +10,16 @@ name = "_core"
 crate-type = ["cdylib", "lib"]
 
 [dependencies]
-# "extension-module" tells pyo3 we want to build an extension module (skips linking against libpython.so)
 # "abi3-py310" tells pyo3 (and maturin) to build using the stable ABI with minimum Python version 3.10
-pyo3 = { version = "0.27.1", features = ["extension-module", "abi3-py310"] }
+# "extension-module" is behind a feature flag so `cargo test` can link against libpython
+pyo3 = { version = "0.27.1", features = ["abi3-py310"] }
 yamlpath = "1"
 yamlpatch = "1"
 # NOTE: serde_yaml 0.9 is archived/unmaintained (dtolnay). No drop-in
 # replacement exists yet. Monitor for a successor or security advisories.
 serde_yaml = "0.9"
 indexmap = "2"
+
+[features]
+default = ["extension-module"]
+extension-module = ["pyo3/extension-module"]