gdb/testsuite: format some Python files

Format with black.

Change-Id: I28e79e9da07ea29391ad1942047633960fa72ed2

gdb/testsuite/gdb.python/py-pp-cast.py

@@ -15,6 +15,7 @@
 
 import gdb.printing
 
+
 class PpIntPrinter(object):
     def __init__(self, val):
         self.val = val

gdb/testsuite/gdb.python/py-pp-re-notag.py

@@ -18,6 +18,7 @@ from time import asctime, gmtime
 import gdb  # silence pyflakes
 import gdb.printing
 
+
 class TimePrinter:
     def __init__(self, val):
         self.val = val

gdb/testsuite/gdb.python/py-prettyprint-stub.py

@@ -18,6 +18,7 @@
 
 import gdb.printing
 
+
 class SPrinter:
     def __init__(self, val):
         self.val = val