Makefile: mrproper should remove *.pyc files

*.pyc files contain compiled Python bytecode. 'make mrproper' should remove
them.

Removing *.pyc files helps for instance sometimes when running into an
error "binman: Unknown entry type 'blob' in node '/binman/blob'".

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>