[tox] Make pass_env compatible with tox4

Change-Id: I2c4e321afe8b5b6bf2e6a03181cd182be06c4f44

tox.ini

@@ -8,7 +8,15 @@ install_command = pip install {opts} {packages}
 basepython = python3
 setenv = VIRTUAL_ENV={envdir}
          PYTHONWARNINGS=default::DeprecationWarning
-passenv = TRACE_FAILONLY GENERATE_HASHES http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
+passenv =
+    TRACE_FAILONLY
+    GENERATE_HASHES
+    http_proxy
+    HTTP_PROXY
+    https_proxy
+    HTTPS_PROXY
+    no_proxy
+    NO_PROXY
 usedevelop = True
 deps = -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
        -r{toxinidir}/requirements.txt