Fix type checker error in discord.__main__

discord/__main__.py

@@ -22,6 +22,9 @@ FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 DEALINGS IN THE SOFTWARE.
 """
 
+from __future__ import annotations
+from typing import Dict, Optional
+
 import argparse
 import sys
 from pathlib import Path
@@ -158,7 +161,7 @@ _cog_extras = '''
 # certain file names and directory names are forbidden
 # see: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247%28v=vs.85%29.aspx
 # although some of this doesn't apply to Linux, we might as well be consistent
-_base_table = {
+_base_table: Dict[str, Optional[str]] = {
     '<': '-',
     '>': '-',
     ':': '-',