Consider bumping MSRV to 1.34.0 for TryFrom

[lopopolo]

I think the only dependency on num-traits in chrono is in the format module which uses ToPrimitive for fallible integer casts.

This functionality is provided by core as e.g. i32::try_from(x).

I hope with the MSRV bump, the dependency on num-traits can be dropped.

[djc]

We're considering bumping to 1.48 soon (unless we get feedback that people still need something older), we can eliminate the num-traits usage then, too (would be nice!).

[esheppa]

MSRV is now 1.38 since #756 - but I'll leave this open so that I don't forget to remove num-traits as we can now use TryFrom

[djc]

Note that we cannot get rid of num-traits in 0.4.x, as it constitutes public API. We could still add TryFrom impls, of course.

[pitdicker]

We now Have TryFrom and the MSRV is 1.56, so this issue seems completed.