fix manifest
Partial implementation of Python USE flag

Nearly works, but still need to set EPYTHON.  Output below:

 * Patching Python site-packages directory
 * ERROR: sci-biology/micro-manager-1.4.13 failed (prepare phase):
 *   python_export: no impl nor EPYTHON
 *
 * Call stack:
 *     ebuild.sh, line   93:  Called src_prepare
 *   environment, line 5249:  Called python_get_sitedir
 *   environment, line 4982:  Called python_export 'PYTHON_SITEDIR'
 *   environment, line 4855:  Called die
 * The specific snippet of code:
 *               [[ -n ${impl} ]] || die "python_export: no impl nor EPYTHON"