Update visa

Update VISA 1.10.1 -> 1.12.0 and pyvisa-py (0.4.1 -> 0.5.3). Other libraries were updated as well.

Note the library changed its import name (visa -> pyvisa). Backward compatibility is ensured everywhere through

try:
    import pyvisa
except ImportError:
    import visa as pyvisa

Support for conda is dropped. The preferred method is now using python's venv module (also provides better explanation in README.