travis: disable dependency installation in "-dev" Python builds (currently fails for Py3.7, and might fail again in the future).

Partial workaround for Github Issue #1852.

Evaluate and join more f-string constants at compile time, including unprefixed strings.
Fix a case where f-string constants could have incorrect constant compile time result values, thus leading to incorrect comparisons and constant folding errors.

Implement two more special cases in fstring joining to reduce the need for runtime character size detection.

Add wheelhouse directories to gitignore.

Remove support and special handling code for Py3.2.

Try to include some more test dependencies in the travis build (but exclude them in Py2.6, which is often no longer supported already).

Include PySet_Check and PyFrozenSet_Check to CPython includes file

Repair some issues with coroutine exception handling

Disable refnanny in PyPy since it otherwise risks to access partly initialised objects, e.g. tuples.

Only reset the frame backlink of *internally* created exception tracebacks in generators/coroutines, not the ones stored away from the caller state.