Remove Python 3.7 branches (#11238)

stdlib/lib2to3/pygram.pyi

@@ -1,5 +1,3 @@
-import sys
-
 from .pgen2.grammar import Grammar
 
 class Symbols:
@@ -112,6 +110,5 @@ class pattern_symbols(Symbols):
 
 python_grammar: Grammar
 python_grammar_no_print_statement: Grammar
-if sys.version_info >= (3, 8):
-    python_grammar_no_print_and_exec_statement: Grammar
+python_grammar_no_print_and_exec_statement: Grammar
 pattern_grammar: Grammar