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.

Can you please review this asap as in today? I need this change for refactoring the pypcc container.

approved this merge request

mentioned in commit e3a7499c

merged