revert hard-code cython version and use env as workaround

Author: oraluben

tests/integration/internal_rpath/pyproject.toml

@@ -1,3 +1,3 @@
 [build-system]
-requires = ["cython < 3.1.0", "setuptools"]
+requires = ["cython", "setuptools"]
 build-backend = "setuptools.build_meta"

tests/integration/multiple_top_level/pyproject.toml

@@ -1,3 +1,3 @@
 [build-system]
-requires = ["cython < 3.1.0", "setuptools"]
+requires = ["cython", "setuptools"]
 build-backend = "setuptools.build_meta"

tests/integration/sample_extension/pyproject.toml

@@ -1,3 +1,3 @@
 [build-system]
-requires = ["setuptools >= 45.0.0", "cython < 3.1.0"]
+requires = ["setuptools >= 45.0.0", "cython"]
 build-backend = "setuptools.build_meta"

tests/integration/test_manylinux.py

@@ -458,6 +458,10 @@ def anylinux(
         for key in os.environ:
             if key.startswith("COV_CORE_"):
                 env[key] = os.environ[key]
+        # cython 3.1.0+ requires a C99 compiler,
+        # where manylinux_2_5 uses gcc 4.8 which is not C99 compliant by default.
+        if policy == "manylinux_2_5":
+            env["CFLAGS"] = "-std=c99"
 
         with docker_container_ctx(manylinux_img, io_folder, env) as container:
             platform_tag = ".".join(