Drop support for Python 3.8, matplotlib 3.4, scipy 1.7, numpy 1.21, Qt5

[mfisher87]

For rationale: https://scientific-python.org/specs/spec-0000/

• Python 3.8
• matplotlib 3.4
• scipy 1.7
• numpy 1.21
• Qt5

[mfisher87]

Should require colorspacious >=1.1.2 (~=1.1?) as well.

[mfisher87]

Qt5 support ended around two weeks ago.

[mfisher87]

FWIW PyQt6 is not yet available through conda-forge, but PySide6 is.