fix: build test_setuptools_abi out of source tree

WHen setuptools-scm is installed, building this package in the source tree with
Python 3.8 on Ubuntu breaks.

Author: Ben Rowland

tests/test_setuptools_abi3.py

@@ -25,7 +25,12 @@
 def test_abi3_wheel(tmp_path, monkeypatch, virtualenv):
     dist = tmp_path / "dist"
     dist.mkdir()
-    monkeypatch.chdir(ABI_PKG)
+
+    # create a temporary copy of the package source so we don't contaminate the
+    # main source tree with build artefacts
+    src = tmp_path / "src"
+    shutil.copytree(ABI_PKG, src)
+    monkeypatch.chdir(src)
     if Path("dist").is_dir():
         shutil.rmtree("dist")