Bug 1918249 - Fix `mach lint` linting prefs JS files on Windows by forcing UTF-8. r=ahal

Prefs files in general are allowed to not be UTF-8, but all the prefs
files in tree are UTF-8, so we can lint them with that assumption.

Differential Revision: https://phabricator.services.mozilla.com/D221878

tools/lint/libpref/__init__.py

@@ -33,7 +33,7 @@ def get_names(pref_list_filename):
     # pattern does not happen in 'name', so it's fine to ignore these.
     # We also want to evaluate all branches of #ifdefs for pref names, so we
     # ignore anything else preprocessor related.
-    file = open(pref_list_filename).read().replace("@", "")
+    file = open(pref_list_filename, encoding="utf-8").read().replace("@", "")
     try:
         pref_list = yaml.safe_load(file)
     except (IOError, ValueError) as e:
@@ -77,7 +77,7 @@ def check_value_for_pref(some_pref, some_value, path):
 # matching to load in prefs.
 def read_prefs(path):
     prefs = []
-    with open(path) as source:
+    with open(path, encoding="utf-8") as source:
         for lineno, line in enumerate(source, start=1):
             match = PATTERN.match(line)
             if match: