Only import importlib_metadata when needed

Similar to #395, this helps with bootstrap issues with python < 3.8.
Adjust test_pip_needs_upgrade_mac_os_11 accordingly.

src/build/env.py

@@ -18,11 +18,6 @@
 import build
 
 
-if sys.version_info < (3, 8):
-    import importlib_metadata as metadata
-else:
-    from importlib import metadata
-
 try:
     import virtualenv
 except ModuleNotFoundError:
@@ -260,6 +255,11 @@ def _create_isolated_env_venv(path: str) -> Tuple[str, str]:
 
     import packaging.version
 
+    if sys.version_info < (3, 8):
+        import importlib_metadata as metadata
+    else:
+        from importlib import metadata
+
     venv.EnvBuilder(with_pip=True, symlinks=_fs_supports_symlink()).create(path)
     executable, script_dir, purelib = _find_executable_and_scripts(path)
 

tests/test_env.py

@@ -134,7 +134,8 @@ def test_pip_needs_upgrade_mac_os_11(mocker, pip_version, arch):
     mocker.patch('platform.system', return_value='Darwin')
     mocker.patch('platform.machine', return_value=arch)
     mocker.patch('platform.mac_ver', return_value=('11.0', ('', '', ''), ''))
-    mocker.patch('build.env.metadata.distributions', return_value=(SimpleNamespace(version=pip_version),))
+    metadata_name = 'importlib_metadata' if sys.version_info < (3, 8) else 'importlib.metadata'
+    mocker.patch(metadata_name+'.distributions', return_value=(SimpleNamespace(version=pip_version),))
 
     min_version = Version('20.3' if arch == 'x86_64' else '21.0.1')
     with build.env.IsolatedEnvBuilder():