chore: rework single python tests (#1835)

* chore: rework single python tests

* chore(tests): limit number of builds in test_abi3

* chore(tests): limit number of builds in test_before_all

* apply review suggestion

use `os.path.samefile` rather than comparing `os.stat` directly.

test/test_build_skip.py

@@ -6,12 +6,12 @@ from . import test_projects, utils
 
 project_with_skip_asserts = test_projects.new_c_project(
     setup_py_add=textwrap.dedent(
-        r"""
-        # explode if run on PyPyor Python 3.7 (these should be skipped)
+        rf"""
         if sys.implementation.name != "cpython":
             raise Exception("Only CPython shall be built")
-        if sys.version_info[0:2] == (3, 7):
-            raise Exception("CPython 3.7 should be skipped")
+        expected_version = {"({}, {})".format(*utils.SINGLE_PYTHON_VERSION)}
+        if sys.version_info[0:2] != expected_version:
+            raise Exception("CPython {{}}.{{}} should be skipped".format(*sys.version_info[0:2]))
         """
     )
 )
@@ -21,17 +21,19 @@ def test(tmp_path):
     project_dir = tmp_path / "project"
     project_with_skip_asserts.generate(project_dir)
 
+    skip = " ".join(
+        f"cp3{minor}-*" for minor in range(6, 30) if (3, minor) != utils.SINGLE_PYTHON_VERSION
+    )
+
     # build the wheels
     actual_wheels = utils.cibuildwheel_run(
         project_dir,
         add_env={
             "CIBW_BUILD": "cp3*-*",
-            "CIBW_SKIP": "cp37-*",
+            "CIBW_SKIP": f"*t-* {skip}",
         },
     )
 
-    # check that we got the right wheels. There should be no PyPy or 3.7.
-    expected_wheels = [
-        w for w in utils.expected_wheels("spam", "0.1.0") if ("-cp3" in w) and ("-cp37" not in w)
-    ]
+    # check that we got the right wheels. There should be a single version of CPython.
+    expected_wheels = utils.expected_wheels("spam", "0.1.0", single_python=True)
     assert set(actual_wheels) == set(expected_wheels)