🐛 FIX: set myst-anchor read/write encoding as utf8 (#610)

Ideally it would read from sphinx `source_encoding`,
but this should fix most use cases, with the default encoding.

myst_parser/cli.py

@@ -13,14 +13,14 @@ def print_anchors(args=None):
     arg_parser.add_argument(
         "input",
         nargs="?",
-        type=argparse.FileType("r"),
+        type=argparse.FileType("r", encoding="utf8"),
         default=sys.stdin,
         help="Input file (default stdin)",
     )
     arg_parser.add_argument(
         "-o",
         "--output",
-        type=argparse.FileType("w"),
+        type=argparse.FileType("w", encoding="utf8"),
         default=sys.stdout,
         help="Output file (default stdout)",
     )