Add links for typeshed PR numbers in old changelog entries

[donbarbos]

Inspired by #180

I used next script:

import os
import re

def add_pr_links_to_changelog(content: str) -> str:
    # See https://github.com/typeshed-internal/stub_uploader/blob/main/stub_uploader/update_changelog.py#L101
    return re.sub(r"\(#(\d+)\)", r"([#\1](https://github.com/python/typeshed/pull/\1))", content)


if __name__ == "__main__":
    directory_path = "./data/changelogs"
    for filename in os.listdir(directory_path):
        file_path = os.path.join(directory_path, filename)

        if filename.endswith(".md"):
            with open(file_path, "r", encoding="utf-8") as file:
                content = file.read()

            updated_content = add_pr_links_to_changelog(content)

            with open(file_path, "w", encoding="utf-8") as file:
                file.write(updated_content)