2021 02 09 master libi2c import only on linux

This adds Linux platform specific installation and import of libi2c.

The requirements.txt file contains now a platform specifier for the libi2c module. The file i2c/I2C.py imports the libi2c module only if it is executed on Linux.

i2c/I2C.py

-import pylibi2c;
+if os.sys.platform is 'linux':
+    import pylibi2c;
 import time
 import logging
 #bus = pylibi2c.I2CDevice('/dev/i2c-1'
@@ -25,7 +26,7 @@ def I2C1server(addr,data,reg=None,read=0):
              bus.ioctl_write(0,str(bytearray([reg])))
          data[:]=[int(x) for x in bus.ioctl_read(0,length)]
        else:
-            if reg is None: 
+            if reg is None:
                bus.iaddr_bytes=0
                reg=0;
             bus.ioctl_write(reg,str(bytearray(data)))