Refactor for newer releases of python and pyvisa

[battlecoder]

(First, thank you so much for writing this library)
Fixed a few things so it works with the Python 3.9 and pyVisa 1.13
Changes are as follow:

• Fixed several print statements that had no parentheses. They are mandatory in Python 3
• Fixed illegal concat of string and byte in one of the print statements
• rigol_ds1054z constructor now receives the instrument resource name as a parameter. This allows the library script to remain unchanged and potentially control multiple devices at once (not tested)
• Updated reference to "visa". Now it's pyvisa