Not sure I could or should limit to v2 only

But since composer and packagist insist on choosing,
I believe this should be a safe choice.
Not changing any GPL mentions in the code.

composer.json

@@ -8,7 +8,7 @@
 		"mailbox"
 	],
 	"homepage" : "http://pf4public.github.io/fetchmail/",
-	"license" : "GPL",
+	"license" : "GPL-2.0-only",
 	"authors" : [{
 			"name" : "Arthur Mayer",
 			"role" : "Retired developer",