test_pip_install: py3.7 unrecognized arguments

py3.7 pip doesn't appear to support requirement specification without a space
between the `-r` and the path

tests/tox_env/python/pip/test_pip_install.py

@@ -313,26 +313,26 @@ def constrained_mock_project(
         exp_constraints.append(requirement)
     elif request.param == "requirements":
         files["requirements.txt"] = f"--pre\n{requirement}"
-        deps = "-rrequirements.txt"
+        deps = "-r requirements.txt"
         exp_constraints.append(requirement)
     elif request.param == "constraints":
         files["constraints.txt"] = constraint
-        deps = "-cconstraints.txt"
+        deps = "-c constraints.txt"
         exp_constraints.append(constraint)
     elif request.param == "explicit+requirements":
         files["requirements.txt"] = f"--pre\n{requirement}"
-        deps = "\n\t-rrequirements.txt\n\tfoo"
+        deps = "\n\t-r requirements.txt\n\tfoo"
         exp_constraints.extend(["foo", requirement])
     elif request.param == "requirements_indirect":
         files["foo.requirements.txt"] = f"--pre\n{requirement}"
-        files["requirements.txt"] = "-rfoo.requirements.txt"
-        deps = "-rrequirements.txt"
+        files["requirements.txt"] = "-r foo.requirements.txt"
+        deps = "-r requirements.txt"
         exp_constraints.append(requirement)
     elif request.param == "requirements_constraints_indirect":
         files["foo.requirements.txt"] = f"--pre\n{requirement}"
         files["foo.constraints.txt"] = f"{constraint}"
-        files["requirements.txt"] = "-rfoo.requirements.txt\n-cfoo.constraints.txt"
-        deps = "-rrequirements.txt"
+        files["requirements.txt"] = "-r foo.requirements.txt\n-c foo.constraints.txt"
+        deps = "-r requirements.txt"
         exp_constraints.extend([requirement, constraint])
     else:  # pragma: no cover
         pytest.fail(f"Missing case: {request.param}")