aio-libs / yarl / 37359747651

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Reject an empty host for http and friends in WHATWG mode (#1970)

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## What do these changes do?

The WHATWG host parser fails on an empty host for every special scheme
except `file`, so `http://`, `http:///`, `http://?q` and `http://#` are
invalid URLs for the WHATWG URL Standard (and for Node's `URL`). WHATWG
mode accepted them. The root cause was printing: a URL of these schemes
without an authority, such as `http:?q` (kept as a reference to a base
for `join()`), was printed with `//` as `http://?q`, so the parser had
to accept `http://?q` to read it back.

WHATWG mode now prints such a URL without `//` (`http:`, `http:/`,
`http:?q`), compares it the same way, and rejects an empty authority for
`http`, `https`, `ws`, `wss` and `ftp` when parsing a string and when an
RFC 3986 mode URL is moved into WHATWG mode. `file` URLs still always
print `//`. RFC 3986 mode is unchanged.

This is a prerequisite for applying RFC 9110 section 4.2.1 (an http URI
with an empty host is invalid) in the RFC 3986 oracle of the conformance
report. With that oracle change alone, `http://?` and `http://#` would
land in "yarl differs, RFC 3986+3987 and WHATWG agree".

Conformance report: `http://?` and `http://#` moved from "WHATWG mode
differs from WHATWG, RFC mode follows RFC 3986+3987" to "yarl follows
each standard in its mode". `http:` against base
`https://example.org/foo/bar` stays in "WHATWG mode differs from
WHATWG", with yarl WHATWG mode now giving `http:` instead of `http://`
(WHATWG fails there; that is the join of a special-scheme reference
against a base with another scheme and is left for a later change). No
other row changed and "yarl differs, RFC 3986+3987 and WHATWG agree"
stays empty.

Existing tests updated because they pinned the old behaviour:
`tests/test_url_special_authority.py::test_kept_without_authority`
(WHATWG column for `http:`, `http:/`, `http:?q`; the `http://`,
`http:///` and `http:///?q` cases moved ... (continued)

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