Python small fixes

[tamarinvs19]

Description

A few small fixes in UTBot Python:

1. fixed empty and one-element tuple code generation
2. fixed missing imports for constructors
3. fixed disconnecting socket for coverage
4. fixed wrong strings with \ at the end
5. fixed serialisation of Infinity from fuzzer
6. fixed serialisation of state field in reduce object
7. fixed focused item in dialog window

How to test

Manual tests

1. generate tests for function which returns empty or one-element tuple, expected: no syntax error

def f(x): return tuple()
def g(x): return (x,)

2. generate tests for utbot-python/samples/samples/using_collections.py, expected: no syntax error
3. no stop generation after some exception in execution
4. no message about in in logs
5. Infinity from fuzzer serialised to float('inf')
6. no problem with serialisation of composite objects
7. try to generate tests for one function from the file with other functions or for one method from the class with other methods, expected: only tested function / method is selected in dialog window.

Self-check list

Check off the item if the statement is true. Hint: [x] is a marked item.

Please do not delete the list or its items.

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.