Allow legacy timezone

[jaakkor2]

My localzone() shows Europe/Kiev that is a legacy name for Europe/Kyiv.

julia> localzone()
Europe/Kiev (UTC+2/UTC+3)

julia> istimezone(string(localzone()))
false

julia> istimezone("Europe/Kyiv")
true

julia> istimezone("Europe/Kiev")
false

julia> istimezone(string(localzone()), TimeZones.Class(:DEFAULT) | TimeZones.Class(:LEGACY))
true

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c1ea853) 84.31% compared to head (2695600) 84.31%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #34 +/- ## ======================================= Coverage 84.31% 84.31% ======================================= Files 4 4 Lines 357 357 ======================================= Hits 301 301 Misses 56 56 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaCon/JuliaCon.jl/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaCon) | Coverage Δ | | |---|---|---| | [src/preferences.jl](https://app.codecov.io/gh/JuliaCon/JuliaCon.jl/pull/34?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaCon#diff-c3JjL3ByZWZlcmVuY2VzLmps) | `100.00% <100.00%> (ø)` | |

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