Fix error while issuing DeprecationWarning for __multicall__ usage

[nicoddemus]

Fix #75

[nicoddemus]

Tests failed because I used the context manager version of pytest.deprecated_call in my test, and that is not available in older pytest versions.

But made me wonder, what's the point of running the pluggy test suite using older pytest versions? Up until now pytest vendored pluggy anyway. Should we just test using the latest pytest, or at least the last known good version?

[goodboy]

@nicoddemus i'm cool with always using latest pytest 👍