refactor library utility more pythonic

[Reskov]

• use .get instead of in and [] preventing extra dictionary lookup
• use f-strings (since we support only 3.8+) reduce string concatenation overhead
• use walrus operator (3.8+) remove extra getattr access in case raw_field found

[alexanderjordanbaker]

@Reskov Are there any performance or other reasons for performing this change besides style?

[Reskov]

This is more about style, rather the performance. For example, in +[] will get one extra dictionary lookup, so as general rule it is better to use get in such case. The same true about getattr, or f-string. But the impact of this is very low, so I am not sure that we will notice this in practice.