Race condition: collection error when directory is deleted before `scandir()` runs

[Zac-HD]

Our scandir() function crashes if the directory to scan does not exist, which can happen if e.g. a tempory directory is deleted during collection (as in HypothesisWorks/hypothesis#4200). I therefore propose catching the directory-not-found error and returning [] in this case:

try:
    with os.scandir(path) as s:
        scanned = list(s)
except FileNotFoundError:
    return []
# Skip entries with symlink loops and other brokenness, so the caller
# doesn't have to deal with it.
for entry in scanned:
    ...