Drop Py37, improve typing and docstrings (#544)

Drops Py3.7, improves typing and adds a mypy config, improves the docstrings and isorts the imports.

* configure isort
* sort with isort
* add github action
* enable flake8-docstrings
* fix docstrings
* add mypy env
* no implicit optional
* add type for raw
* add some type hints

setup.cfg

@@ -31,7 +31,7 @@ project_urls =
     Release Notes = https://github.com/python-escpos/python-escpos/releases
 
 [options]
-python_requires = >=3.6
+python_requires = >=3.8
 zip_safe = false
 include_package_data = true
 install_requires =
@@ -46,7 +46,6 @@ install_requires =
     PyYAML
     argparse
     argcomplete
-    future
     importlib_resources
 setup_requires = setuptools_scm
 tests_require =
@@ -65,4 +64,3 @@ tests_require =
 exclude = .git,.tox,.github,.eggs,__pycache__,doc/conf.py,build,dist,capabilities-data,test,src/escpos/constants.py
 max-line-length = 120
 extend-ignore = E203, W503
-# future-imports = absolute_import, division, print_function, unicode_literals  # we are not there yet