Fix selection of type-inference strategy.

Fix a bug in the type inference regarding cases where not type information is present.

Add a PyLint checker for calls to print().

Extend the blacklist of modules that shall not be analysed.

Raise RuntimeError from tracer when called from another thread.

Provide better exception messages for critical failures.

Apply a further limit to the execution time of a single generated test case to at most 10 seconds.

Exclude empty enum classes from test cluster to fix test generation.

Parsing included modules raised an issue when the enum module is used: the test cluster then had a reference to the enum.Enum class, which obviously does not contain any fields. In the following, generating tests failed, as soon as this class was selected to fulfil parameter values because there was no field to select from, e.g., MyEnum.MY_FIELD. We now exclude empty enums from the test cluster.