gnu: Beets: Fix compatibility with Python 3.7.

* gnu/packages/patches/beets-python-3.7-fix.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
*  gnu/packages/music.scm (beets)[source]: Use it.

gnu/packages/patches/beets-python-3.7-fix.patch

@@ -0,0 +1,57 @@
+Fix compatibility issue with Python 3.7:
+
+https://github.com/beetbox/beets/issues/2978
+
+Patch copied from upstream source repository:
+
+https://github.com/beetbox/beets/commit/15d44f02a391764da1ce1f239caef819f08beed8
+
+From 15d44f02a391764da1ce1f239caef819f08beed8 Mon Sep 17 00:00:00 2001
+From: Adrian Sampson <adrian@radbox.org>
+Date: Sun, 22 Jul 2018 12:34:19 -0400
+Subject: [PATCH] Fix Python 3.7 compatibility (#2978)
+
+---
+ beets/autotag/hooks.py | 8 +++++++-
+ docs/changelog.rst     | 2 ++
+ 2 files changed, 9 insertions(+), 1 deletion(-)
+
+diff --git a/beets/autotag/hooks.py b/beets/autotag/hooks.py
+index 3615a9333..1c62a54c5 100644
+--- a/beets/autotag/hooks.py
++++ b/beets/autotag/hooks.py
+@@ -31,6 +31,12 @@
+ 
+ log = logging.getLogger('beets')
+ 
++# The name of the type for patterns in re changed in Python 3.7.
++try:
++    Pattern = re._pattern_type
++except AttributeError:
++    Pattern = re.Pattern
++
+ 
+ # Classes used to represent candidate options.
+ 
+@@ -433,7 +439,7 @@ def _eq(self, value1, value2):
+         be a compiled regular expression, in which case it will be
+         matched against `value2`.
+         """
+-        if isinstance(value1, re._pattern_type):
++        if isinstance(value1, Pattern):
+             return bool(value1.match(value2))
+         return value1 == value2
+ 
+#diff --git a/docs/changelog.rst b/docs/changelog.rst
+#index be6de2904..d487f31f5 100644
+#--- a/docs/changelog.rst
+#+++ b/docs/changelog.rst
+#@@ -19,6 +19,8 @@ New features:
+# 
+# Fixes:
+# 
+#+* Fix compatibility Python 3.7 and its change to a name in the ``re`` module.
+#+  :bug:`2978`
+# * R128 normalization tags are now properly deleted from files when the values
+#   are missing.
+#   Thanks to :user:`autrimpo`.