clases/lib: Use modern exception syntax

Update older code to use modern exception handling syntax which
is the form accepted by python 3.

(From OE-Core rev: b010501cd089e649a68f683be0cf4d0aac90fbe3)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/base.bbclass

@@ -108,7 +108,7 @@ python base_do_fetch() {
     try:
         fetcher = bb.fetch2.Fetch(src_uri, localdata)
         fetcher.download()
-    except bb.fetch2.BBFetchException, e:
+    except bb.fetch2.BBFetchException as e:
         raise bb.build.FuncFailed(e)
 }
 
@@ -128,7 +128,7 @@ python base_do_unpack() {
     try:
         fetcher = bb.fetch2.Fetch(src_uri, localdata)
         fetcher.unpack(rootdir)
-    except bb.fetch2.BBFetchException, e:
+    except bb.fetch2.BBFetchException as e:
         raise bb.build.FuncFailed(e)
 }