Fixes issues #38 #39: Save initial gpio Line_Request object, do not release gio lines at every gpio action

[wke67]

• The code for Class LoRaGpio is using a with statement to do operations on a gpio line. This will always request the gpio line, do some operation, and release the gpio line thereafter. This will have the effect of resetting the event counter every time monitor() is called. Instead, the gpio line must be requested once, the Line_Request object saved, and used for all future accesses to the gpio line. The same has been done for the input and output methods of class LoRaGpio. This fixes issues LoRaGpio: method monitor_continuous(...) not implemented #38, Class LoRaGpio method monitor(...) does not work as expected. #39.
• A change in SX127x.py allows to set the chip-select (cs) gpio line to None - many drivers/devices provide a fixed chip-select line and a gpio is not required.
• Updated requirements in setup.cfg