Specify environment-conditional transitive deps.

Workaround for pypa/pip#12770.

Author: jaraco

tools/vendored.py

@@ -219,6 +219,11 @@ def install_deps(deps, vendor):
     """
     Install the deps to vendor.
     """
+    # workaround for https://github.com/pypa/pip/issues/12770
+    deps += [
+        'zipp >= 3.7',
+        'backports.tarfile',
+    ]
     install_args = [
         sys.executable,
         '-m',