Fix test_test_command_install_requirements with pip 20.3+

setuptools appends --find-links to pip if dependency_links is found, and
both takes URLs [1][2].

[1] https://github.com/pypa/setuptools/blob/v51.0.0/docs/userguide/dependency_management.rst#dependencies-that-arent-in-pypi
[2] https://pip.pypa.io/en/stable/reference/pip_install/#cmdoption-f

setuptools/tests/test_virtualenv.py

@@ -2,6 +2,8 @@
 import os
 import sys
 
+import pathlib
+
 import pytest
 from pytest import yield_fixture
 from pytest_fixture_config import yield_requires_config
@@ -124,7 +126,7 @@ def sdist(distname, version):
         make_nspkg_sdist(str(dist_path), distname, version)
         return dist_path
     dependency_links = [
-        str(dist_path)
+        pathlib.Path(str(dist_path)).as_uri()
         for dist_path in (
             sdist('foobar', '2.4'),
             sdist('bits', '4.2'),