* Remove from __future__ import print_function, because we
  no longer support Python2.
* Clean out unused parts of tools/scripts/scriptCommon.py
* Move appveyorMergeCoverageScript to Python3
* Update user reporting in *release scripts
* Cleanup module imports

Signed-off-by: Alan Jowett <alanjo@microsoft.com>

Also modified the AppVeyor script to account for changes in what
happens to PATH with the new installer.

Closes #2059

There are two reasons for this:

1) It is highly unlikely that someone has use for this header,
which has no customization points and only provides simplest
possible main, and cannot link the static library which also
provides a default main implementation.
2) It being a header was causing extra complications with
the convenience headers, and our checking script. This would either
require special handling in the checking script, or would break user's
of the main convenience header.

All in all, it is simpler and better in the long term to remove it,
than to fix its problems.

Development build enables warnings and and `Werror` or equivalent.

Previously, we would collect coverage data for all source files in
Catch2's directory, including tests and examples, and we would then
ask codecov.io to ignore those. With this change, OpenCppCoverage
only collects coverage data for source files in the `src/` directory.
This cuts the size of the coverage report in half, and also speeds
up the coverage collection.