As explained in Codex
post__not_in(array) – use post ids. Specify post NOT to retrieve.
post__not_in argument have to be passed as array.
Change your query to:
$the_query = new WP_Query( array(
'category__in' => $project_category,
'posts_per_page' => -1,
'post__not_in' => array( $postid ),
) );
And it should work.
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