Add pattern to replace if-else block with dict.get() when possible

[vlopezferrando]

Join multiple with statements into a single one. Rule SIM401 from flake8-simplify.

Caveat: if either the key or the default value are functions, they will be called a different
number of times in the generated code. We may need to enforce that $key and $default don't
call any function.

[morgante]

Let's handle that in the code. You can use a where clause.

[vlopezferrando]

Done, I have added:

 where {
    !$key <: contains call(),
    !$default <: contains call(),
}

Of course, in Python it is tricky to 100% enforce this, as an operator could be overloaded with anything.