Add workaround for editable failure

See https://projects.torsion.org/witten/borgmatic/pulls/165#issuecomment-1416.

tox.ini

@@ -7,6 +7,9 @@ minversion = 3.10.0
 usedevelop = True
 deps = -rtest_requirements.txt
 whitelist_externals = sh
+install_command =
+    # temporary work around for https://github.com/pypa/pip/issues/6434
+    python -m pip install --no-use-pep517 {opts} {packages}
 commands =
     pytest
     sh scripts/black