wmllint: Leave gzipped binary files alone, they're not safe for consumption

Fixes wmllint crashing like this upon encountering a gzip tarball in an
add-on:

  Traceback (most recent call last):
    File "/home/shadowm/bin/wmllint-1.14", line 3188, in <module>
      for fn in allcfgfiles(directory):
    File "/home/shadowm/bin/wmllint-1.14", line 2944, in allcfgfiles
      if interesting(os.path.join(root, name)):
    File "/home/shadowm/bin/wmllint-1.14", line 2927, in interesting
      return fn.endswith(".cfg") or is_map(fn) or issave(fn)
    File "/home/shadowm/src/wesnoth-1.14/data/tools/wesnoth/wmltools3.py", line 270, in issave
      return firstline.startswith("label=")
  TypeError: startswith first arg must be bytes or a tuple of bytes, not str

[ci skip]

changelog.md

@@ -84,6 +84,7 @@
      selection dialog is dismissed.
    * Fixed an issue with positioned sound sources ignoring the volume set in
      Preferences after going off the audible radius and back (issue #3280).
+   * Fixed wmllint choking on gzipped binary files (e.g. gzipped tarballs).
 
 ## Version 1.14.3
  ### AI

data/tools/wesnoth/wmltools3.py

@@ -260,6 +260,9 @@ def issave(filename):
     if filename.endswith(".gz"):
         with gzip.open(filename) as content:
             firstline = content.readline()
+            if not isinstance(firstline, str):
+                # It's a compressed binary file
+                return False
     else:
         try:
             with codecs.open(filename, "r", "utf8") as content: