[API] Fix Python Examples (#4943)

[API] Fix Python Examples (#4943)

When testing the API examples, Python examples were not included. This
commit changes that and fixes multiple minor issues that came up once
the tests were enabled:

- It adds `Solver::supportsFloatingPoint()` as an API method that
  returns whether CVC4 is configured to support floating-point numbers
  or not (this is useful for failing gracefully when floating-point
  support is not available, e.g. in the case of our floating-point
  example).
- It updates the `expections.py` example to use the new API.
- It fixes the `sygus-fun.py` example. The example was passing a _set_
  of non-terminals to `Solver::mkSygusGrammar()` but the order in which
  the non-terminals are passed in matters because the first non-terminal
  is considered to be the starting terminal. The commit also updates the
  documentation of that function.
- It fixes the Python API for datatypes. The `__getitem__` function had
  a typo and the `datatypes.py` example was crashing as a result.