Let unpaper overwrite temporary files.

I'm not sure what the circumstances are, but it looks like unpaper can
attempt to write a temporary file that already exists [0]. This then
fails the consumption. As per daedadu's comment simply letting unpaper
overwrite files fixes this.

[0]
unpaper: error: output file '/tmp/paperless/paperless-pjkrcr4l/convert-0000.unpaper.pnm' already present.
See https://web.archive.org/web/20181008081515/https://github.com/danielquinn/paperless/issues/406#issue-360651630

src/paperless_tesseract/parsers.py

@@ -218,7 +218,8 @@ def run_convert(*args):
 
 def run_unpaper(args):
     unpaper, pnm = args
-    command_args = unpaper, pnm, pnm.replace(".pnm", ".unpaper.pnm")
+    command_args = (unpaper, "--overwrite", pnm,
+                    pnm.replace(".pnm", ".unpaper.pnm"))
     if not subprocess.Popen(command_args).wait() == 0:
         raise ParseError("Unpaper failed at {}".format(command_args))