Fix MSRV check by pinning once_cell (#484)

Author: tuxu

.github/workflows/ci.yml

@@ -274,6 +274,7 @@ jobs:
           import toml
           cargo_toml = toml.load("Cargo.toml")
           cargo_toml["dependencies"]["ndarray"] = "0.15.6"
+          cargo_toml["dependencies"]["once_cell"] = "1.20.3"
           with open("Cargo.toml", "w") as f:
             toml.dump(cargo_toml, f)
         working-directory: examples/simple
@@ -290,6 +291,8 @@ jobs:
             pkg_id = pkg["name"] + ":" + pkg["version"]
             if pkg["name"] == "ndarray" and pkg["version"] != "0.15.6":
               subprocess.run(["cargo", "update", "--package", pkg_id, "--precise", "0.15.6"], check=True)
+            elif pkg["name"] == "once_cell" and pkg["version"] != "1.20.3":
+              subprocess.run(["cargo", "update", "--package", pkg_id, "--precise", "1.20.3"], check=True)
         working-directory: examples/simple
         shell: python
       - name: Test example