Fix faulty istree deprecation

[termi-official]

This should essentially fix SciML/ModelingToolkit.jl#2827 . The issue was that istree has been marked as deprecated, but istree is also the same function as iscall, causing Aqua to fail.

[github-actions]

Benchmark Results

	master	9541a58...	master/9541a581d14ec9...
overhead/acrule/a+2	0.74 ± 0.024 μs	0.737 ± 0.021 μs	1
overhead/acrule/a+2+b	0.715 ± 0.021 μs	0.712 ± 0.019 μs	1
overhead/acrule/a+b	0.252 ± 0.01 μs	0.259 ± 0.0082 μs	0.973
overhead/acrule/noop:Int	25.9 ± 0.92 ns	26.2 ± 0.05 ns	0.988
overhead/acrule/noop:Sym	0.0366 ± 0.0055 μs	0.0368 ± 0.005 μs	0.995
overhead/rule/noop:Int	0.0443 ± 0.0014 μs	0.0446 ± 0.0019 μs	0.994
overhead/rule/noop:Sym	0.0556 ± 0.0035 μs	0.0557 ± 0.0031 μs	0.999
overhead/rule/noop:Term	0.0552 ± 0.0031 μs	0.0557 ± 0.0033 μs	0.991
overhead/ruleset/noop:Int	0.131 ± 0.0039 μs	0.131 ± 0.0032 μs	1.01
overhead/ruleset/noop:Sym	0.154 ± 0.0054 μs	0.154 ± 0.0062 μs	1
overhead/ruleset/noop:Term	3.76 ± 0.15 μs	3.67 ± 0.14 μs	1.03
overhead/simplify/noop:Int	0.158 ± 0.0039 μs	0.155 ± 0.0021 μs	1.02
overhead/simplify/noop:Sym	0.156 ± 0.0052 μs	0.15 ± 0.0028 μs	1.04
overhead/simplify/noop:Term	0.0376 ± 0.002 ms	0.0392 ± 0.0022 ms	0.957
overhead/simplify/randterm (+, *):serial	0.092 ± 0.0021 s	0.0946 ± 0.0019 s	0.973
overhead/simplify/randterm (+, *):thread	0.0544 ± 0.032 s	0.0545 ± 0.035 s	0.998
overhead/simplify/randterm (/, *):serial	0.225 ± 0.0087 ms	0.227 ± 0.0077 ms	0.992
overhead/simplify/randterm (/, *):thread	0.259 ± 0.0094 ms	0.259 ± 0.0089 ms	1
overhead/substitute/a	0.0576 ± 0.0016 ms	0.0558 ± 0.0017 ms	1.03
overhead/substitute/a,b	0.0509 ± 0.0016 ms	0.0485 ± 0.0016 ms	1.05
overhead/substitute/a,b,c	17.9 ± 0.83 μs	16.6 ± 0.9 μs	1.08
polyform/easy_iszero	0.0343 ± 0.0021 ms	0.0338 ± 0.0021 ms	1.01
polyform/isone	2.79 ± 0.01 ns	2.79 ± 0.009 ns	1
polyform/iszero	1.25 ± 0.039 ms	1.23 ± 0.036 ms	1.02
polyform/simplify_fractions	1.87 ± 0.049 ms	1.82 ± 0.043 ms	1.03
time_to_load	4.65 ± 0.071 s	4.59 ± 0.015 s	1.01

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[termi-official]

IntegrationTest / Symbolics.jl/All (pull_request) also fails on master.