CVS loader: UnicodeEncodeError when calling 'rcsparse.rcsfile' when files are not valid UTF-8

https://sentry.softwareheritage.org/organizations/swh/issues/9008/?referrer=phabricator_plugin

UnicodeEncodeError: 'utf-8' codec can't encode character '\udce4' in position 54: surrogates not allowed
  File "swh/loader/core/loader.py", line 335, in load
    self.prepare()
  File "swh/loader/cvs/loader.py", line 415, in prepare
    rcsfile = rcsparse.rcsfile(filepath)  # noqa: F841

Loading failure, updating to `failed` status

List of origins causing this:

• rsync://a.cvs.sourceforge.net/cvsroot/dynamicdraw/Doc/
• rsync://a.cvs.sourceforge.net/cvsroot/maklerdesign/Kunde/
• rsync://a.cvs.sourceforge.net/cvsroot/maklerdesign/Zwischenpraesentation_FS2/
• rsync://a.cvs.sourceforge.net/cvsroot/smtpsvr/bbs/
• rsync://a.cvs.sourceforge.net/cvsroot/aspintranet/aspintranet/
• rsync://a.cvs.sourceforge.net/cvsroot/stricqv2/stricqv2/
• rsync://a.cvs.sourceforge.net/cvsroot/epgsukapa/epgret/
• rsync://a.cvs.sourceforge.net/cvsroot/keepalivexp/kAllStatus/
• rsync://a.cvs.sourceforge.net/cvsroot/soliton/soliton/
• rsync://a.cvs.sourceforge.net/cvsroot/yast/Dokumente/

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Migrated from T3980 (view on Phabricator)