python: Apply patch for CVE-2019-20907

Incluing the patch file in-tree because the upstream patch is not
intended to apply for Python 2.

Re #94004

pkgs/development/interpreters/python/cpython/2.7/CVE-2019-20907.patch

···
+From 5a8d121a1f3ef5ad7c105ee378cc79a3eac0c7d4 Mon Sep 17 00:00:00 2001
+From: Rishi <[e-mail]>
+Date: Wed, 15 Jul 2020 13:51:00 +0200
+Subject: [PATCH] bpo-39017: Avoid infinite loop in the tarfile module
+ (GH-21454)
+
+Avoid infinite loop when reading specially crafted TAR files using the tarfile module
+(CVE-2019-20907).
+---
+ Lib/tarfile.py                                    |   2 ++
+
+diff --git a/Lib/tarfile.py b/Lib/tarfile.py
+index e2b60532f6..6769066cab 100755
+--- a/Lib/tarfile.py
++++ b/Lib/tarfile.py
+@@ -1249,6 +1249,8 @@ class TarInfo(object):
+ 
+             length, keyword = match.groups()
+             length = int(length)
++            if length == 0:
++                raise InvalidHeaderError("invalid header")
+             value = buf[match.end(2) + 1:match.start(1) + length - 1]
+ 
+             # Normally, we could just use "utf-8" as the encoding and "strict"

pkgs/development/interpreters/python/cpython/2.7/default.nix

···
 
       # Backport from CPython 3.8 of a good list of tests to run for PGO.
       ./profile-task.patch
+
+      # Patch is likely to go away in the next release (if there is any)
+      ./CVE-2019-20907.patch
     ] ++ optionals (x11Support && stdenv.isDarwin) [
       ./use-correct-tcl-tk-on-darwin.patch
     ] ++ optionals stdenv.isLinux [