Fix HTTP 429 from pypistats.org by adding User-Agent header

[Copilot]

The fetch.py script was failing with HTTP Error 429: Too Many Requests when querying the pypistats.org API, because urllib sends no meaningful User-Agent by default and pypistats.org rate-limits such requests.

Changes

• fetch.py: Replace bare urlopen(url) in count_pypi() with a Request object that includes a User-Agent header.

# Before
pypi_api = urlopen(f"https://pypistats.org/api/packages/{package}/overall")

# After
req = Request(
    f"https://pypistats.org/api/packages/{package}/overall",
    headers={"User-Agent": "fetch-download-stats/1.0"},
)
pypi_api = urlopen(req)