Consistency: use utf-8 encoding name everywhere

In fact, Python codec is `utf_8` but it has aliases, `utf8' (declared as
alias) or `utf-8` (because `_` or `-` do not make difference) are
working, but we mainly use `utf-8` among the code.

See https://docs.python.org/3/library/codecs.html#standard-encodings

picard/script/serializer.py

@@ -222,7 +222,7 @@ def export_script(self, parent=None):
         else:
             script_text = self.script + "\n"
         try:
-            with open(filename, 'w', encoding='utf8') as o_file:
+            with open(filename, 'w', encoding='utf-8') as o_file:
                 o_file.write(script_text)
         except OSError as error:
             raise ScriptImportExportError(format=FILE_ERROR_EXPORT, filename=filename, error_msg=error.strerror)
@@ -252,7 +252,7 @@ def import_script(cls, parent=None):
             return None
         log.debug("Importing script file: %s", filename)
         try:
-            with open(filename, 'r', encoding='utf8') as i_file:
+            with open(filename, 'r', encoding='utf-8') as i_file:
                 file_content = i_file.read()
         except OSError as error:
             raise ScriptImportExportError(format=FILE_ERROR_IMPORT, filename=filename, error_msg=error.strerror)

picard/tagger.py

@@ -615,7 +615,7 @@ def handle_command_submit_fingerprints(self, argstring):
 
     def handle_command_write_logs(self, argstring):
         try:
-            with open(argstring, 'w', encoding='utf8') as f:
+            with open(argstring, 'w', encoding='utf-8') as f:
                 for x in self.window.log_dialog.log_tail.contents():
                     f.write(f"{x.message}\n")
         except Exception as e:
@@ -1533,7 +1533,7 @@ def main(localedir=None, autoupdate=True):
         identifier = uuid4().hex
     else:
         if picard_args.config_file:
-            identifier = blake2b(picard_args.config_file.encode('utf8'), digest_size=16).hexdigest()
+            identifier = blake2b(picard_args.config_file.encode('utf-8'), digest_size=16).hexdigest()
         else:
             identifier = 'main'
         if picard_args.no_plugins: