DifferentiationInterface-v0.3.4

What's Changed

• Separate tests by backend by @gdalle in #244
• Sort tests into single and double backends by @gdalle in #245
• Move coloring soundness tests to DIT by @gdalle in #246
• Add Symbolics to backend docs by @gdalle in #247
• Clean up tests by @gdalle in #248
• Julia 1.6 compat by @gdalle in #243
• Fix warning indentation by @gdalle in #250

Full Changelog: DifferentiationInterface-v0.3.3...DifferentiationInterface-v0.3.4