setup.cfg: update pyvisa version constraint

[MatthieuDartiailh]

• [x] Closes #266
• [x] Executed black . && isort -c . && flake8 with no errors
• [x] Added an entry to the CHANGES file

[MatthieuDartiailh]

bors r+

[bors[bot]]

Build succeeded:

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• pyvisa.pyvisa-py.keysight-assisted
• Tests result