gnu: Python: fix the compilation of some modules from the standard library.

This commit enables the bz2, dbm, readline, ssl and zlib modules.

* gnu/packages/gdbm.scm: Enable the compatibility mode.
* gnu/packages/python.scm: Enable a few modules from the standard library.
* gnu/packages/patches/python-fix-dbm.patch: New file.
* Makefile.am: Add it.