use `===` for `Symbol`s

[t-bltg]

$ grep -irl ' == :' | xargs sed -i 's, == :, === :,g'
$ grep -irl ' != :' | xargs sed -i 's, != :, !== :,g'

[codecov[bot]]

Codecov Report

Base: 71.90% // Head: 74.38% // Increases project coverage by +2.47% :tada:

Coverage data is based on head (86cafff) compared to base (da407a3). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #104 +/- ## ========================================== + Coverage 71.90% 74.38% +2.47% ========================================== Files 1 1 Lines 121 121 ========================================== + Hits 87 90 +3 + Misses 34 31 -3 ``` | [Impacted Files](https://codecov.io/gh/JuliaGraphics/ColorSchemes.jl/pull/104?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGraphics) | Coverage Δ | | |---|---|---| | [src/ColorSchemes.jl](https://codecov.io/gh/JuliaGraphics/ColorSchemes.jl/pull/104?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGraphics#diff-c3JjL0NvbG9yU2NoZW1lcy5qbA==) | `74.38% <100.00%> (+2.47%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGraphics). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGraphics)

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[cormullion]

Cool, thanks!