pylint: treat tz as a good name

`tz` is a part of the Datetime public API and we don't plan to change it
now.

Part of #270

Author: DifferentialOrange

.pylintrc

@@ -1,4 +1,4 @@
 [BASIC]
 
 # Good variable names which should always be accepted, separated by a comma
-good-names=i,j,k,ex,Run,_,ok,t
+good-names=i,j,k,ex,Run,_,ok,t,tz