GeezerVison

…sis stubs

Hypothesis lets a test pin a known-failing input with
`@example(...).xfail(reason=..., raises=...)`, which is the natural way
to record a bug found by a property test without taking the input out of
the strategy or writing a separate expectedFailure test. The stub
`example` is a plain decorator function, so any test using `.xfail()` or
`.via()` failed to import when Hypothesis is not installed.

This turns the stub into a class with those two methods. An expected
failure is strict, as in Hypothesis: when the examples are replayed the
test must raise one of the `raises` exceptions for that input, and an
unexpected pass fails the test. `.via()` is a no-op, as it is upstream.

…he line

When an encoded word does not fit in the remaining space of a line,
_fold_as_ew shrinks the chunk being encoded one character at a time
until it fits. Nothing stops it from shrinking the chunk to nothing,
and an empty chunk makes no progress: the loop appends an empty
encoded word and a new continuation line forever.

This is reachable whenever a single character's encoded form is wider
than a fresh continuation line, e.g. any character outside the BMP
with max_line_length=20 (the q-encoded word is 24 characters). The two
earlier fixes in this area (bpo-33529, bpo-36564) handled lines too
short for the RFC 2047 chrome, but not lines too short for the chrome
plus one character.

This keeps at least one character in the chunk, so the loop always
advances and the line runs over max_line_length instead. That matches
what _refold_parse_tree already does when it cannot fold a part at
all, and avoids raising for widths that work for every other input.

The regression test pins the exact output, as the earlier two did. The
Hypothesis property checks that folding any unstructured value at any
width either raises HeaderParseError (only possible at widths of 13 or
fewer, where no encoded word fits) or reparses to the same value modulo
leading whitespace, which the parser drops.

…ok-alikes

The round trip property cannot cover text that is itself a well-formed
encoded word, because such text is written out unencoded and decodes to
something else on reparse. That is now tracked separately as pythongh-160148,
so point at it from the strategy rather than carrying an expected
failure here.

…the round trip

The folding round-trip strategy cannot produce a well-formed encoded
word, because such text is written out verbatim and decoded on reparse
(pythongh-160148), and until now the only record of that in the test was a
comment. With example.xfail() in the stubs (pythongh-160356), three such
subjects are now expected-failure examples on the round-trip test, so
the test fails again once pythongh-160148 is resolved and the carve-out can
go.