Fix some PEP-8 violations in Python code

[orsinium]

PEP-8 is an optional (but strongly recommended) code style advised by the Python core dev team for Python code. This PR fixes some of the PEP-8 violations. It's hard to fix all of them, though. For instance, PEP-8 recommends avoiding wildcard imports but Z3 heavily relies on them.

Anyway, this is what is fixed:

1. Bare except: block replaced by more precise exceptions handled (except Exception:)
2. Wrap too long lines. PEP-8 advises 80 chars line length limit but I decided to be less strict and wrap everything to 120 chars.
3. Make some isinstance check shorter: use isinstance(a, (int, str)) instead of isinstance(a, int) or isinstance(a, str)
4. Autoformat code using autopep8 in aggressive mode: drop trailing whitespaces, separate top-level declarations by 2 blank lines, use a is None instead of a == None, wrap binary operations with single whitespaces, use 4 whitespaces for indentation, start comments from single #.
5. Autoformat code using unify: use double quotes for all string literals for consistency.
6. Use sys.version_info instead of sys.version.

There are still remain a few violations reported by flake8 but they are potential bugs and so require a more thoughtful approach. This PR is already huge, so I want to keep it simple and safe, with mostly stylistic changes.

[ghost]

All CLA requirements met.

[orsinium]

Thank you!