python.o is installed by "make libainstall". It is done since 2001 (commit 85515ad9795ffc3b676cbddeeea2b003818a2623).

Git history:

commit 49fd7fa4431da299196d74087df4a04f99f9c46f
Author: Thomas Wouters <thomas@python.org>
Date:   Fri Apr 21 10:40:58 2006 +0000

    Merge p3yk branch with the trunk up to revision 45595. This breaks a fair
    number of tests, all because of the codecs/_multibytecodecs issue described
    here (it's not a Py3K issue, just something Py3K discovers):
    http://mail.python.org/pipermail/python-dev/2006-April/064051.html
    (...)

-       $(INSTALL_DATA) Modules/$(MAINOBJ) $(DESTDIR)$(LIBPL)/$(MAINOBJ)
+       $(INSTALL_DATA) Modules/python.o $(DESTDIR)$(LIBPL)/python.o

commit a1a84e7d4f7b493bb6fa5415ce55d3f722221470
Author: Fred Drake <fdrake@acm.org>
Date:   Tue Mar 6 05:52:16 2001 +0000

    Move all knowledge that $(MAINOBJ) is built in the Modules/ directory
    into Makefile.pre.in; the configure script will only determine the basename
    of the file.
    
    This fixes installation of a Python built using C++, reported by Greg
    Wilson.

-       $(INSTALL_DATA) Modules/python.o $(LIBPL)/python.o
+       $(INSTALL_DATA) Modules/$(MAINOBJ) $(LIBPL)/$(MAINOBJ)

commit 85515ad9795ffc3b676cbddeeea2b003818a2623
Author: Neil Schemenauer <nascheme@enme.ucalgary.ca>
Date:   Wed Jan 24 17:11:43 2001 +0000

    Flat makefile based on toplevel Makefile.in and makefiles in build
    subdirectories.  Those other makefiles will go away eventually.

+       $(INSTALL_DATA) Modules/python.o $(LIBPL)/python.o