Skip AppVeyor builds on non-code changes

Remove the old get-pip.py shim

Add a new tools/ directory and move some files into it

Start warning on non compliant pyproject.toml files and rejecting obviously incorrect ones

Update doc and comment URLs to https where available

Document the output format of pip show

The installed set of packages is already fixed and no longer changing.
It is therefore safe to reuse a single `pkg_resources.WorkingSet` for
all invocations of `get_installed_version`.

Restrict install time dependency warnings to directly-dependant packages

Reorder methods in req_install to better group them

Remove useless manifest entries

These were made redundant since those files have been removed.

* Rename dev-requirements to tests-requirements

Miscellaneous minor cleanups

This makes no functional difference, it's just cleaner.

Remove pytest version constraint for CI