set py2_import directive so imports in Cython source code not forced to absolute

Cython/Compiler/Code.py

-# cython: language_level = 3
+# cython: language_level = 3, py2_import=True
 #
 #   Pyrex - Code output module
 #

Cython/Compiler/Lexicon.py

-# cython: language_level=3
+# cython: language_level=3, py2_import=True
 #
 #   Cython Scanner - Lexical Definitions
 #

Cython/Compiler/Parsing.py

-# cython: auto_cpdef=True, infer_types=True, language_level=3
+# cython: auto_cpdef=True, infer_types=True, language_level=3, py2_import=True
 #
 #   Pyrex Parser
 #

Cython/Compiler/Scanning.py

-# cython: infer_types=True, language_level=3
+# cython: infer_types=True, language_level=3, py2_import=True
 #
 #   Cython Scanner
 #