gnu: tremc: Fix execution with recent Python.

* gnu/packages/bittorrent.scm (tremc)[source]: Add patch.
* gnu/packages/patches/tremc-fix-decodestring.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.

Reported by gnoo in #guix

gnu/local.mk

@@ -1857,6 +1857,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/tor-sandbox-i686.patch			\
   %D%/packages/patches/transcode-ffmpeg.patch	\
   %D%/packages/patches/transmission-honor-localedir.patch	\
+  %D%/packages/patches/tremc-fix-decodestring.patch		\
   %D%/packages/patches/trytond-add-egg-modules-to-path.patch	\
   %D%/packages/patches/trytond-add-guix_trytond_path.patch	\
   %D%/packages/patches/ttf2eot-cstddef.patch			\

gnu/packages/bittorrent.scm

@@ -231,7 +231,8 @@ XML-RPC over SCGI.")
        (file-name (git-file-name name version))
        (sha256
         (base32
-         "1fqspp2ckafplahgba54xmx0sjidx1pdzyjaqjhz0ivh98dkx2n5"))))
+         "1fqspp2ckafplahgba54xmx0sjidx1pdzyjaqjhz0ivh98dkx2n5"))
+       (patches (search-patches "tremc-fix-decodestring.patch"))))
     (build-system gnu-build-system)
     (arguments
      `(#:tests? #f                      ; no test suite

gnu/packages/patches/tremc-fix-decodestring.patch

@@ -0,0 +1,25 @@
+From 79995bc82e81f0429222aab4d90a03440feef057 Mon Sep 17 00:00:00 2001
+From: Matan Ziv-Av <matan@svgalib.org>
+Date: Mon, 29 Jun 2020 15:18:03 +0300
+Subject: [PATCH] Use base64.decodebytes.
+
+Based on upstream commit 79995bc82e81f0429222aab4d90a03440feef057 which
+confuses several unrelated changes.
+
+---
+ tremc | 29 ++++++++++++++++++++++++++++-
+ 1 file changed, 28 insertions(+), 1 deletion(-)
+
+diff --git a/tremc b/tremc
+index b47f012..c626fde 100755
+--- a/tremc
++++ b/tremc
+@@ -440,7 +443,7 @@ class Transmission(object):
+                 # TAG_TORRENT_DETAILS, but just passing seems to help.(?)
+                 try:
+                     torrent_details = response['arguments']['torrents'][0]
+-                    torrent_details['pieces'] = base64.decodestring(bytes(torrent_details['pieces'], ENCODING))
++                    torrent_details['pieces'] = base64.decodebytes(bytes(torrent_details['pieces'], ENCODING))
+                     self.torrent_details_cache = torrent_details
+                     self.upgrade_peerlist()
+                 except IndexError: