[youtube] non-ascii characters in channel name ignored, leading to breakage

[hseg]

$ youtube-dl --config-location /dev/null -v https://www.youtube.com/c/Pawe%C5%82Zadro%C5%BCniak
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--config-location', '/dev/null', '-v', 'https://www.youtube.com/c/Pawe%C5%82Zadro%C5%BCniak']
[debug] Encodings: locale UTF-8, fs utf-8, out utf-8, pref UTF-8
[debug] youtube-dl version 2020.07.28
[debug] Python version 3.8.5 (CPython) - Linux-5.8.3-arch1-1-x86_64-with-glibc2.2.5
[debug] exe versions: ffmpeg 4.3.1, ffprobe 4.3.1
[debug] Proxy map: {}
[youtube:user] Pawe: Downloading channel page
WARNING: Unable to download webpage: HTTP Error 404: Not Found
[youtube:user] Pawe: Downloading page #1
ERROR: Unable to download webpage: HTTP Error 404: Not Found (caused by <HTTPError 404: 'Not Found'>); please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see  https://yt-dl.org/update  on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
  File "/usr/lib/python3.8/site-packages/youtube_dl/extractor/common.py", line 627, in _request_webpage
    return self._downloader.urlopen(url_or_request)
  File "/usr/lib/python3.8/site-packages/youtube_dl/YoutubeDL.py", line 2238, in urlopen
    return self._opener.open(req, timeout=self._socket_timeout)
  File "/usr/lib/python3.8/urllib/request.py", line 531, in open
    response = meth(req, response)
  File "/usr/lib/python3.8/urllib/request.py", line 640, in http_response
    response = self.parent.error(
  File "/usr/lib/python3.8/urllib/request.py", line 563, in error
    result = self._call_chain(*args)
  File "/usr/lib/python3.8/urllib/request.py", line 502, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.8/urllib/request.py", line 755, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
  File "/usr/lib/python3.8/urllib/request.py", line 531, in open
    response = meth(req, response)
  File "/usr/lib/python3.8/urllib/request.py", line 640, in http_response
    response = self.parent.error(
  File "/usr/lib/python3.8/urllib/request.py", line 569, in error
    return self._call_chain(*args)
  File "/usr/lib/python3.8/urllib/request.py", line 502, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.8/urllib/request.py", line 649, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)

Note in particular the attempt to access https://youtube.com/c/Pawe
Note: am running on https://github.com/alexmerkel/youtube-dl/tree/webpfix (#25717), rebased to 2020.07.08, but I don't think the changes involved broke this.