Adding Symbolics.jl as a weakdep for SpecialFunctionsSymbolicsExt #525

Job	Run time
Documentation	59s
Julia 1.3 - ubuntu-latest - x64	13s
Julia 1.3 - macOS-latest - x64	36s
Julia 1.3 - windows-latest - x64	35s
Julia 1 - ubuntu-latest - x64	2m 28s
Julia 1 - macOS-latest - x64	3m 1s
Julia 1 - windows-latest - x64	3m 7s
Julia nightly - ubuntu-latest - x64	3m 17s
Julia nightly - macOS-latest - x64	3m 57s
Julia nightly - windows-latest - x64	4m 12s
	22m 25s

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