"""Serialize bytes into a base64 encoded string, with optional formatting.

This function encodes the input bytes into a base64 string. If the `format` parameter is set to "base64url", the output is modified to be URL-safe by stripping padding characters and replacing '+' with '-' and '/' with '_'.

Args:
    o (bytes): The bytes to be serialized.
    format (Optional[str]): The format of the output. If "base64url", the output is URL-safe. Defaults to None.

Returns:
    str: The base64 encoded string, optionally formatted for URL safety.
"""
You should write code starting with:
import base64
import typing

def _serialize_bytes(o: bytes, format: typing.Optional[str] = None) -> str:
    # TODO: Implement this function
    pass

