weave: use explicit pdf.close(), drastically reduce open file handles

With the new pikepdf 1.2.0 we no longer need to hold file handles
open because of the "copy to memory" functionality. We retain
the behavior of closing/reopening the output PDF every 100 pages as
a way to limit memory usage.

tests/test_weave.py

@@ -53,7 +53,7 @@ def test_no_glyphless_weave(resources, outdir):
     pdf.save(outdir / 'test.pdf')
 
     env = os.environ.copy()
-    env['_OCRMYPDF_MAX_OPEN_PAGE_PDFS'] = '2'
+    env['_OCRMYPDF_MAX_REPLACE_PAGES'] = '2'
     check_ocrmypdf(
         outdir / 'test.pdf',
         outdir / 'out.pdf',