python3: add patch to fix PEP 3147 issue with automake built packages

Packages built with automake use a `py-compile` helper to byte-compile
Python source files. This script uses the "py_compile" module from the
standard library. In turn, the compile() function in the "py_compile"
module invokes the cache_from_source() function provided by importlib.

This commit adds a new patch named "020-importlib-no-pep3147.patch"
that changes cache_from_source() and source_from_cache() in importlib
to get rid of the "__pycache__" directory.

This commit fixes the following import error in kmod when the module
is built for Python 3:

  >>> from kmod import Kmod
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  ImportError: cannot import name 'Kmod'

Moreover, this commit removes two patches that are no longer necessary
since modifying cache_from_source() and source_from_cache() disables
PEP 3147 for the standard library and distutils / setuptools.

 * 004-old-stdlib-cache.patch
 * 016-distutils-no-pep3147.patch

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/python3/python3.mk

@@ -147,7 +147,17 @@ define PYTHON3_TOUCH_GRAMMAR_FILES
 	touch $(@D)/Include/graminit.h $(@D)/Python/graminit.c
 endef
 
+# This prevents the Python Makefile from regenerating the
+# Python/importlib.h header if Lib/importlib/_bootstrap.py has changed
+# because its generation is broken in a cross-compilation environment
+# and importlib.h is not used.
+
+define PYTHON3_TOUCH_IMPORTLIB_H
+	touch $(@D)/Python/importlib.h
+endef
+
 PYTHON3_POST_PATCH_HOOKS += PYTHON3_TOUCH_GRAMMAR_FILES
+PYTHON3_POST_PATCH_HOOKS += PYTHON3_TOUCH_IMPORTLIB_H
 
 #
 # Remove useless files. In the config/ directory, only the Makefile