Backport: Fix `maximum(abs, ::Fun)`

[codecov[bot]]

Codecov Report

Patch coverage: 33.33% and project coverage change: -1.23% :warning:

Comparison is base (b1beea8) 70.60% compared to head (7ba179a) 69.37%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## backport_0.8 #571 +/- ## ================================================ - Coverage 70.60% 69.37% -1.23% ================================================ Files 81 81 Lines 8584 8540 -44 ================================================ - Hits 6061 5925 -136 - Misses 2523 2615 +92 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/571?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/specialfunctions.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/571?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL3NwZWNpYWxmdW5jdGlvbnMuamw=) | `68.09% <33.33%> (-0.99%)` | :arrow_down: | ... and [38 files with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/571/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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