diff -ur astropy-4.2/astropy/coordinates/angle_utilities.py astropy-4.2.ply/astropy/coordinates/angle_utilities.py
--- astropy-4.2/astropy/coordinates/angle_utilities.py	2020-11-25 08:28:48.000000000 +0100
+++ astropy-4.2.ply/astropy/coordinates/angle_utilities.py	2021-02-15 23:22:45.935721825 +0100
@@ -88,7 +88,7 @@
 
     @classmethod
     def _make_parser(cls):
-        from astropy.extern.ply import lex, yacc
+        from ply import lex, yacc
 
         # List of token names.
         tokens = (
diff -ur astropy-4.2/astropy/units/format/cds.py astropy-4.2.ply/astropy/units/format/cds.py
--- astropy-4.2/astropy/units/format/cds.py	2020-11-25 08:28:48.000000000 +0100
+++ astropy-4.2.ply/astropy/units/format/cds.py	2021-02-15 23:22:29.190476911 +0100
@@ -82,7 +82,7 @@
     @classmethod
     def _make_lexer(cls):
 
-        from astropy.extern.ply import lex
+        from ply import lex
 
         tokens = cls._tokens
 
@@ -154,7 +154,7 @@
         <https://bitbucket.org/nxg/unity/>`_.
         """
 
-        from astropy.extern.ply import yacc
+        from ply import yacc
 
         tokens = cls._tokens
 
diff -ur astropy-4.2/astropy/units/format/generic.py astropy-4.2.ply/astropy/units/format/generic.py
--- astropy-4.2/astropy/units/format/generic.py	2020-11-25 08:28:48.000000000 +0100
+++ astropy-4.2.ply/astropy/units/format/generic.py	2021-02-15 23:21:39.375748327 +0100
@@ -118,7 +118,7 @@
 
     @classmethod
     def _make_lexer(cls):
-        from astropy.extern.ply import lex
+        from ply import lex
 
         tokens = cls._tokens
 
@@ -198,7 +198,7 @@
         formats, the only difference being the set of available unit
         strings.
         """
-        from astropy.extern.ply import yacc
+        from ply import yacc
 
         tokens = cls._tokens
 
diff -ur astropy-4.2/astropy/units/format/ogip.py astropy-4.2.ply/astropy/units/format/ogip.py
--- astropy-4.2/astropy/units/format/ogip.py	2020-11-25 08:28:48.000000000 +0100
+++ astropy-4.2.ply/astropy/units/format/ogip.py	2021-02-15 23:22:06.526145425 +0100
@@ -111,7 +111,7 @@
 
     @classmethod
     def _make_lexer(cls):
-        from astropy.extern.ply import lex
+        from ply import lex
 
         tokens = cls._tokens
 
@@ -186,7 +186,7 @@
         <https://bitbucket.org/nxg/unity/>`_.
         """
 
-        from astropy.extern.ply import yacc
+        from ply import yacc
 
         tokens = cls._tokens