v0.9.21

ApproxFunBase v0.9.21

Diff since v0.9.20

Merged pull requests:

• Backport: Remove explicit test on v1.10 (#616) (@jishnub)
• Test on v1.9 and v1 (#617) (@jishnub)
• backport: Three-term ldiv for UpperTriangular AlmostBandedMatrix (#618) (@jishnub)
• Dev ApproxFunBaseTest in downstream tests (#619) (@jishnub)
• Backport: Dev ApproxFunBaseTest in downstream tests (#620) (@jishnub)
• Fix type instability in constructors.jl (#621) (@jondeuce)
• Bump version to v0.9.21 (#622) (@jishnub)