Remove usage of parser module, deprecated in Python 3.9

Fix #6404

Author: nicoddemus

changelog/6404.trivial.rst

@@ -0,0 +1 @@
+Remove usage of ``parser`` module, deprecated in Python 3.9.

src/_pytest/_code/source.py

@@ -123,15 +123,12 @@ def isparseable(self, deindent=True):
         """ return True if source is parseable, heuristically
             deindenting it by default.
         """
-        from parser import suite as syntax_checker
-
         if deindent:
             source = str(self.deindent())
         else:
             source = str(self)
         try:
-            # compile(source+'\n', "x", "exec")
-            syntax_checker(source + "\n")
+            ast.parse(source)
         except KeyboardInterrupt:
             raise
         except Exception: