Gostcrypto

- Added Python version checking (use version 3.6 or higher)

- Use ``**kvargs`` in the ``new`` function with default parameters (**gostrandom**, **gosthash**, **gosthmac**, **gostpbkdf**)
- Add the ability to pass data to the ``new`` function from **gosthmac**
- Fixed some minor bugs in the **gostrandom** module

- Fixed a default initial vector bug (added default initial vector for 'magma' algorithm)

- Fixed a MAC calculation bug when using padding in the **gostcipher** module (in earlier versions (including 1.2.1), the MAC with padding was calculated incorrectly (the bug was in the 'msb' and 'update' functions))

- Refactoring code **gostcipher** module (changed the class structure)
- Each module has its own exception class added
- In the ``new`` function of the **gostcipher** module for MAC mode, it is now possible to pass data for MAC calculation, followed by calling the ``digest`` method without first calling the ``update`` method
- In the ``new`` function of the **gosthash** module, it is now possible to pass data for hash calculation, followed by calling the ``digest`` method without first calling the ``update`` method
- Added new exceptions for various conflict situations
- Fixed some minor bugs

- Fixed a MAC calculation bug when using padding in the **gostcipher** module