support for regex-syntax 0.7

Hi,

In debian we are in the process of upgrading the regex crates, while we do have a mechanism for packaging multiple versions of the same crate it is something we try to avoid where possible. regex itself is not semver-breaking, but some of the related crates are.

I tried bumping the regex-syntax dependency in sequoia-openpgp, but it failed with a whole bunch of errors, the first few of which are posted below. in particular regex-syntax has bumped semver from 0.6 to 0.7

error[E0433]: failed to resolve: could not find `GroupKind` in `hir`
    --> /debcargo-conf/build/sequoia-openpgp/target/debug/build/sequoia-openpgp-eb0302473e0c2d48/out/regex/grammar.rs:2364:28
     |
2364 |                 kind: hir::GroupKind::NonCapturing,
     |                            ^^^^^^^^^ could not find `GroupKind` in `hir`

error[E0433]: failed to resolve: could not find `RepetitionKind` in `hir`
    --> /debcargo-conf/build/sequoia-openpgp/target/debug/build/sequoia-openpgp-eb0302473e0c2d48/out/regex/grammar.rs:2398:28
     |
2398 |                 kind: hir::RepetitionKind::ZeroOrMore,
     |                            ^^^^^^^^^^^^^^ could not find `RepetitionKind` in `hir`

error[E0433]: failed to resolve: could not find `RepetitionKind` in `hir`   
   --> /debcargo-conf/build/sequoia-openpgp/target/debug/build/sequoia-openpgp-eb0302473e0c2d48/out/regex/grammar.rs:2422:28
     |
2422 |                 kind: hir::RepetitionKind::OneOrMore,
     |                            ^^^^^^^^^^^^^^ could not find `RepetitionKind` in `hir`