Update syntax for tox4

Author: ConorMacBride

tox.ini

@@ -11,7 +11,10 @@ requires =
 isolated_build = true
 
 [testenv]
-passenv = DISPLAY WINDIR MPL_UPDATE_*
+passenv =
+    DISPLAY
+    WINDIR
+    MPL_UPDATE_*
 setenv =
     MPLBACKEND = Agg
 changedir = .tmp/{envname}