":" in relative path within relative reference

'.://' should not parse. It is not an absolute URI because '.' is not a valid scheme, and it is not a relative URI because a path-noscheme cannot begin with a ':'.

The relevant grammar rules from the RFC:

   scheme        = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
   relative-ref  = relative-part [ "?" query ] [ "#" fragment ]
   relative-part = "//" authority path-abempty
                 / path-absolute
                 / path-noscheme
                 / path-empty
   path-noscheme = segment-nz-nc *( "/" segment )
   segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
                 ; non-zero-length segment without any colon ":"

该 issue 未指定文件或测试；首先在 Python 包中找到相对引用解析和验证的入口点。为 '.://' 添加一个回归用例，并根据所引用的 RFC 语法验证它会被拒绝。