Clean implementation of Parser/pgen and fix some style issues (GH-12156)

diff --git a/Parser/pgen/token.py b/Parser/pgen/token.py
index f9d45c4..008e241 100644
--- a/Parser/pgen/token.py
+++ b/Parser/pgen/token.py
@@ -1,5 +1,6 @@
 import itertools
 
+
 def generate_tokens(tokens):
     numbers = itertools.count(0)
     for line in tokens:
@@ -16,6 +17,7 @@
     yield ('N_TOKENS', next(numbers))
     yield ('NT_OFFSET', 256)
 
+
 def generate_opmap(tokens):
     for line in tokens:
         line = line.strip()