add requires for 1.6

[codecov-commenter]

Codecov Report

Patch coverage: 83.33% and project coverage change: +0.19% :tada:

Comparison is base (664d493) 70.54% compared to head (ae309a7) 70.74%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #33 +/- ## ========================================== + Coverage 70.54% 70.74% +0.19% ========================================== Files 25 25 Lines 3758 3763 +5 ========================================== + Hits 2651 2662 +11 + Misses 1107 1101 -6 ``` | [Files Changed](https://app.codecov.io/gh/j-fu/ExtendableGrids.jl/pull/33?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/ExtendableGrids.jl](https://app.codecov.io/gh/j-fu/ExtendableGrids.jl/pull/33?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL0V4dGVuZGFibGVHcmlkcy5qbA==) | `75.00% <66.66%> (-25.00%)` | :arrow_down: | | [ext/ExtendableGridsGmshExt.jl](https://app.codecov.io/gh/j-fu/ExtendableGrids.jl/pull/33?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZXh0L0V4dGVuZGFibGVHcmlkc0dtc2hFeHQuamw=) | `92.30% <100.00%> (+0.09%)` | :arrow_up: | ... and [6 files with indirect coverage changes](https://app.codecov.io/gh/j-fu/ExtendableGrids.jl/pull/33/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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