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.

setup.py

@@ -99,7 +99,7 @@ setup(
         'cffi >= 1.9.1',  # must be a setup and install requirement
         'img2pdf >= 0.3.0, < 0.4',  # pure Python, so track HEAD closely
         'pdfminer.six == 20181108 ; sys_platform != "darwin"',
-        'pikepdf >= 1.1.0, < 2',
+        'pikepdf >= 1.2.0, < 2',
         'Pillow >= 4.0.0, != 5.1.0 ; sys_platform == "darwin"',
         # Pillow < 4 has BytesIO/TIFF bug w/img2pdf 0.2.3
         # block 5.1.0, broken wheels