Deprecated variants broken in nightly

[dtolnay]

As of the most recent nightly, it seems the test from #73 is no longer working.

$ cargo +nightly-2021-06-03 check --test test_deprecated
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s

$ cargo +nightly-2021-06-04 check --test test_deprecated
    Checking thiserror v1.0.25 (/git/thiserror)
error: use of deprecated unit variant `Error::Deprecated`
 --> tests/test_deprecated.rs:9:5
  |
9 |     Deprecated,
  |     ^^^^^^^^^^
  |
note: the lint level is defined here
 --> tests/test_deprecated.rs:1:9
  |
1 | #![deny(deprecated, clippy::all, clippy::pedantic)]
  |         ^^^^^^^^^^