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Cross-building the 3.6 source with PYTHON_FOR_BUILD using archlinux python 3.5.1 as found in PATH, fails with:
_PYTHON_PROJECT_BASE=/home/xavier/src/android/pyona/build/python3.6-android-21-x86 _PYTHON_HOST_PLATFORM=linux-x86 PYTHONPATH=/home/xavier/src/packages/android/cpython/Lib:/home/xavier/src/packages/android/cpython/Lib/plat-i386-linux-gnu python3 -S -m sysconfig --generate-posix-vars ;\
if test $? -ne 0 ; then \
	echo "generate-posix-vars failed" ; \
	rm -f ./pybuilddir.txt ; \
	exit 1 ; \
fi
Could not import runpy module
Traceback (most recent call last):
 File "/home/xavier/src/packages/android/cpython/Lib/runpy.py", line 15, in <module>
 import importlib.util
 File "/home/xavier/src/packages/android/cpython/Lib/importlib/util.py", line 25
 raise ValueError(f'no package specified for {repr(name)} '
 ^
SyntaxError: invalid syntax
generate-posix-vars failed
The reason is that the syntax of formatted string literals is unknown to python 3.5.

yes, I think we have to limit the choice of the interpreter for the build to the same major version.

With this patch, cross-building the 3.6 source with only python 3.5.1 on the PATH, produces the following configure error:
 checking for python interpreter for cross build... configure: error: python3.6 interpreter not found

New patch that gets rid of the changes in configure made by my version of autoconf. This version is 2.69 though, i.e. the same as the generated configure file that is under control of mercurial in the default branch.

restrict_interp_2.patch: LGTM.
not ... == '...' can be written ... != '...', no?

I think the way the condition was written previously is more expressive, so the patch does not change that. The 'not' here expresses the fact that in the shell syntax, the condition is true when the test returns 0 [1], but we still want to express the fact that the 'break' is taken when the interpreter version matches $PACKAGE_VERSION.
[1] https://www.gnu.org/software/bash/manual/html_node/Conditional-Constructs.html#Conditional-Constructs 

New changeset 62802d373e9f by Xavier de Gaye in branch 'default':
Issue #27434: Version of interpreter running a cross-build and source version must be the same.
https://hg.python.org/cpython/rev/62802d373e9f