Raise an ImportError instead of OSError for unsupported macOS versions

docs/source/index.md

@@ -76,6 +76,17 @@ http = urllib3.PoolManager(ssl_context=ctx)
 resp = http.request("GET", "https://example.com")
 ```
 
+If Truststore can't work for a given platform due to APIs not being available then
+at import time the exception `ImportError` will be raised with an informative message:
+
+```python
+# On Python 3.9 and earlier:
+import truststore  # Raises 'ImportError'
+
+# On macOS 10.7 and earlier:
+import truststore  # Raises 'ImportError'
+```
+
 ### Using truststore with pip
 
 [Pip v22.2](https://discuss.python.org/t/announcement-pip-22-2-release/17543) includes experimental support for verifying certificates with system trust stores using `truststore`. To enable the feature, use the flag `--use-feature=truststore` when installing a package like so:

src/truststore/_macos.py

@@ -21,7 +21,7 @@
 _mac_version = platform.mac_ver()[0]
 _mac_version_info = tuple(map(int, _mac_version.split(".")))
 if _mac_version_info < (10, 8):
-    raise OSError(
+    raise ImportError(
         f"Only OS X 10.8 and newer are supported, not {_mac_version_info[0]}.{_mac_version_info[1]}"
     )
 

tests/test_api.py

@@ -1,11 +1,13 @@
 import asyncio
+import importlib
 import os
 import platform
 import socket
 import ssl
 import tempfile
 from dataclasses import dataclass
 from operator import attrgetter
+from unittest import mock
 
 import aiohttp
 import aiohttp.client_exceptions
@@ -324,3 +326,19 @@ def test_trustme_cert_still_uses_system_certs(trustme_ca):
         resp = http.request("GET", "https://example.com")
     assert resp.status == 200
     assert len(resp.data) > 0
+
+
+def test_macos_10_7_import_error():
+    with mock.patch("platform.mac_ver") as mac_ver:
+        # This isn't the full structure, but the version is the first element.
+        mac_ver.return_value = ("10.7",)
+
+        with pytest.raises(ImportError) as e:
+            # We want to force a fresh import, so either we get it on the
+            # first try because the OS isn't macOS or we get it on
+            # the call to importlib.reload(...).
+            import truststore._macos
+
+            importlib.reload(truststore._macos)
+
+        assert str(e.value) == "Only OS X 10.8 and newer are supported, not 10.7"