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As the title states, mkpwent() in pwdmodule.c accesses `pw_gecos`, which is not defined for struct passwd in Bionic, Android's C library.
Attached is a patch that works around the issue by setting the field to None on Android.

Interesting. It seems pw_gecos isn't mandated by POSIX:
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/pwd.h.html
I wonder if there's a better way to do this (autoconf check?) than an Android-specific #ifdef, though.

Ah, yes, if it's not actually mandated by POSIX, something like HAVE_PASSWD_PW_GECOS would be more appropriate. I'll rework the patch into something more generic.

Here is shiz's patch extended with the addition to configure.ac. I added the variable HAVE_PASSWD_GECOS_FIELD and the appropriate tests. Luckily this very problem (missing pw_gecos field) is the example used in the autoconf manual (https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Generic-Structures.html#Generic-Structures). (Otherwise I would have been as lost as I usually am with autoconf.)

Apologies. That last patch includes diffs to generated files (configure and pyconfig.h.in). This version just patches Modules/pwdmodule.c and configure.ac.
After applying the patch please run "autoheader" and "autoconf" to correctly regenerate configure and pyconfig.h.in.

New changeset 0d74d4937ab9 by Stefan Krah in branch 'default':
Issue #20306: The pw_gecos and pw_passwd fields are not required by POSIX.
https://hg.python.org/cpython/rev/0d74d4937ab9 

With changeset 0d74d4937ab9, test_pwd fails on android API level 21 at test_values with:
 Traceback (most recent call last):
 File "/sdcard/org.bitbucket.pyona/lib/python3.6/test/test_pwd.py", line 27, in test_values
 self.assertIsInstance(e.pw_passwd, str)
 AssertionError: None is not an instance of <class 'str'>
On android API 21 HAVE_STRUCT_PASSWD_PW_PASSWD is defined but p->pw_passwd is set to NULL.

New changeset f0f519aca558 by Stefan Krah in branch 'default':
Issue #20306: Android is the only system that returns NULL for the pw_passwd
https://hg.python.org/cpython/rev/f0f519aca558 

Okay, for the record: I think that returning "None" would probably be
more correct than the empty string, but I don't think users actually
care to the point that they will introduce a case split in their
applications.
Realistically, probably no one cares about pw_passwd at all.