Using the mapping protocol for ConfigReader

[Amaras]

The documentation for ConfigParser recommends using the mapping protocol for all new projects instead of the legacy API.

This PR aims to change all cases of this pattern:

section = 'XXX'
# here self is a `ConfigParser` subclass instance
if self.has_section(section):
  key = 'YYY' 
  if self.has_option(section, key):
    var = self.get(section, key)
    # do something with var
  else:
    # log
else:
  # log and return

The goal is to use the following pattern instead:

section_key = 'XXX'
try:
  section = self[section_key]
except KeyError:
  # log and return

key = 'YYY'
try:
  var = section[key]
except TypeError:
  # log and return : not a section
except KeyError:
  # log: no option YYY
# use var

I have also added several comments on parts of the code that could (theoretically) lead to a crash or security vulnerability

[Amaras]

There are some TODOs left, but the small-scale test proved it works