Rename the ExprType "parent" attribute to "parents".
Rename the ExprType "parent" attribute to "parents".

This is fallout from commit 76ca2c0b1527541be59c344118c538ba055ad9d8,
which renamed the *parent parameter of ExprType.__init__, but failed
to rename the instance attribute to which it was assigned.

Signed-off-by: Julien Danjou <5c682c2d1ec4073e277f9ba9f4bdf07e5794dabe@danjou.info>

nbaum/xcb-proto,nbaum/xcb-proto