Fix some python lint issues and reactivate the associated Pylint and Flake8 codes

[GC-repeat]

Several Pylint and Flake8 codes for the linter were disabled when setting the linter up. Some of them can be turned back on while fixing the associated issues:

Flake8

• [x] E226 missing whitespace around arithmetic operator (Flake8)
• [x] E231 missing whitespace after ',' (Flake8)
• [x] E251 unexpected spaces around keyword / parameter equals (Flake8)
• [x] E265 block comment should start with '# '
• [x] E302 expected 2 blank lines, found 1
• [x] E305 expected 2 blank lines after class or function definition, found 1
• [x] W291 trailing whitespace
• [x] E275 missing whitespace after keyword (see #314)
• [x] E501 line too long (>120 characters)
• [x] E203 whitespace before ':'

Pylint

• [x] C0303: Trailing whitespace (trailing-whitespace)
• [x] C0301: Line too long (>120 characters)

[GC-repeat]

Do we want to enable "E203 whitespace before ':'" and the line too long (triggered only above 120) too ? The list of what has been disabled can be found here #204

[GC-repeat]

Fixed with #314 and #322