Restore new resolver for pip. Proper fix for pypa/pip#9143.

Author: jaraco

.github/workflows/main.yml

@@ -17,6 +17,11 @@ jobs:
     runs-on: ${{ matrix.platform }}
     steps:
       - uses: actions/checkout@v2
+        with:
+          # fetch all branches and tags (to get tags for versioning)
+          # ref actions/checkout#448
+          fetch-depth: 0
+
       - name: Setup Python
         uses: actions/setup-python@v2
         with:

tox.ini

@@ -12,9 +12,6 @@ commands =
 	pytest {posargs}
 usedevelop = True
 extras = testing
-setenv =
-	# workaround pypa/pip#9143
-	PIP_USE_DEPRECATED=legacy-resolver
 
 [testenv:docs]
 extras =