Support @example(...).via("string description") to track example provenance

[Zac-HD]

Over in Zac-HD/hypofuzz#13, I mention using magic comments to track which @example() cases were automatically added by the fuzzer, to enably fully-automated maintanence of a static covering set.

It occured to me that provenance tracking might be a more generally desirable feature, and that a simple method which returns self might be a better way to implement this than comments. Once I hit on the short-and-sweet name via, I was sold. Probably requires some use of typing.Protocol to help type-checking tools understand that this is fine, with corresponding compatibility workaround on Python 3.7 (which predates Protocol). The resulting code might look like:

@example(0, 1)  # manually added
@example(1, 2).via("you can put anything you like in here")  # manually added
@example(3, 4).via("hypofuzz")  # added *and removable* by hypofuzz
@given(...)
def test(x: int, y: int):
    pass

[Zac-HD]

Guess what? This is a syntax error before Python 3.9, because it requires PEP-614.

I find the idea of a # via: your string here comment aesthetically offensive, but since we'll be supporting 3.8 until the EOL in October 2025 I guess I'll have to suck it up and deal 😬. Or maybe we just tell people on older versions that they have to write

def fx(dec): return dec

@fx(example(...).via("your string here"))  # look, it's just a function call, no need for PEP-614 here...
def test(_):
    pass

(plus we'll need to update the base Python env if we want to show .via() in docs without breaking anything, etc...)

[Zac-HD]

Closed by #3516.