Use new metadata extracting feature in snaps

This patch replaces the deprecated `version-script` key with new
Snapcraft metadata extracting features based on the `adopt-info` and
`parse-info` keys and the `snapcraftctl set-version` command.

This allows metadata sharing with Setuptools setup.py file.

Refernce:

    Extracting information from sources in snapcraft parts - doc - snapcraft.io
    https://forum.snapcraft.io/t/extracting-information-from-sources-in-snapcraft-parts/4642

Signed-off-by: 林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>

snap/snapcraft.yaml

@@ -1,6 +1,5 @@
 name: glances
-version: set-by-version-script
-version-script: git describe --always --dirty --tags --match 'v*' | sed s/^v//
+adopt-info: glances
 summary: Glances an Eye on your system. A top/htop alternative.
 description: |
   Glances is a cross-platform monitoring tool which aims to present
@@ -32,6 +31,15 @@ parts:
   glances:
     plugin: python
     source: .
+    # FIXME: Theoretically this also replaces `summary` and `description`
+    #        keys, however due to the following bug we still need to keep
+    #        them until it is fixed.
+    #        https://bugs.launchpad.net/snapcraft/+bug/1813364
+    parse-info: [setup.py]
+    override-pull: |
+      snapcraftctl pull
+      snapcraftctl set-version \
+        "$(git describe --always --dirty --tags --match 'v*' | sed s/^v//)"
   bottle:
     plugin: python
     source: https://github.com/bottlepy/bottle.git