Move from the deprecated platform.linux_distribution() to the external package distro

platform.linux_distribution() has been deprecated some time ago, and the
only viable alternative seems to be this external package.
Add it as an optioanl dependency, as without it distribution detection
(i.e. `diffoscope --list-tools`) won't work.

Also add it as a Debian dependency, as it's a very lightway package, and
the usefulness it brings are great.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

debian/control

@@ -45,6 +45,7 @@ Build-Depends:
  python-argcomplete,
  python3-all,
  python3-debian <!nocheck>,
+ python3-distro <!nocheck>,
  python3-docutils,
  python3-guestfs <!nocheck>,
  python3-libarchive-c,
@@ -77,6 +78,7 @@ Breaks:
 Replaces:
  debbindiff (<< 29),
 Depends:
+ python3-distro,
  python3-pkg-resources,
  ${misc:Depends},
  ${python3:Depends},

diffoscope/exc.py

@@ -18,6 +18,11 @@
 # You should have received a copy of the GNU General Public License
 # along with diffoscope.  If not, see <https://www.gnu.org/licenses/>.
 
+try:
+    import distro
+except ImportError:
+    distro = None
+
 from .tools import get_current_os
 from .external_tools import EXTERNAL_TOOLS
 

diffoscope/external_tools.py

@@ -24,7 +24,7 @@ EXTERNAL_TOOLS = {
     'bsdtar': {
         'debian': 'libarchive-tools',
         'arch': 'libarchive',
-        'Fedora': 'bsdtar',
+        'fedora': 'bsdtar',
         'FreeBSD': 'libarchive',
     },
     'bzip2': {

diffoscope/tools.py

@@ -18,8 +18,13 @@
 # along with diffoscope.  If not, see <https://www.gnu.org/licenses/>.
 
 import collections
-import platform
 import functools
+import platform
+
+try:
+    import distro
+except ImportError:
+    distro = None
 
 from distutils.spawn import find_executable
 
@@ -73,7 +78,6 @@ def tool_required(command):
 def get_current_os():
     system = platform.system()
     if system == "Linux":
-        # FIXME: Will break under Python 3.7, see:
-        # https://docs.python.org/3/library/platform.html#platform.linux_distribution
-        return platform.linux_distribution()[0]
+        if distro:
+            return distro.id()
     return system  # noqa

setup.py

@@ -45,6 +45,9 @@ setup(
         'python-magic',
         'libarchive-c',
     ],
+    extras_require={
+        'distro_detection': ['distro'],
+    },
     classifiers=[
         'Development Status :: 3 - Alpha',
         'Intended Audience :: Developers',

tests/comparators/test_binary.py

@@ -167,7 +167,7 @@ def test_with_compare_details_and_tool_not_found(monkeypatch):
         {
             'debian': 'some-package',
             'arch': 'some-package',
-            'Fedora': 'some-package',
+            'fedora': 'some-package',
             'FreeBSD': 'some-package'
         }
     )