Convert to Python3

[douglasdcm]

Hi, congratulations for the tool. To improve the maintenance, in this MR I am:

• Converting the code from Python 2 to Python 3 based on Python 3.7
• Converting string format to f-string to improve the performance
• Adding type hinting to functions and variables. I inspected it with mypy
• Running black formatter
• Adding support to .env file
• Adding .gitignore

Evidences
Unit tests (I was not able to run the Web tests, because I was not able to find the values for environment variables)

nosetests tests/test_cssify.py

----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK

Checking typing consistency

mypy cssify/
Success: no issues found in 2 source files

Execution command line

 python cssify/cssify.py //a
a