meson: Make scripts/version.py work with older Pythons

Older Python versions (3.5, 2.x) do not have a __getitem__ method
on regex match objects, so use the .group() method instead, which is
available in all versions.

(cherry picked from commit 1635540)

Author: aperezdc

scripts/version.py

@@ -18,6 +18,6 @@
     for line in f.readlines():
         m = version_re.match(line)
         if m:
-            version[m[1]] = m[2]
+            version[m.group(1)] = m.group(2)
 
 print("{}.{}.{}".format(version["MAJOR"], version["MINOR"], version["MICRO"]))