scripts: properly check for errors in whitespacecleanup/run-all-cleanup

An error in whitespacecleanup, like a bug in isort that raises an exception,
should be treated as a problem instead of silently ignored.

py3: switch to use Python 3 interpreter, temporarily leaving many things very broken until they have been migrated/fixed in a reviewable way

Bump Mercurial minimum version to 5.2 - the first version that claim stable py3
support.

scripts/validate-commits: use direct /bin/bash interpreter

Use of '#!/bin/bash' vs '#!/usr/bin/env bash' can be argued multiple ways.
For example, see:
https://stackoverflow.com/questions/21612980/why-is-usr-bin-env-bash-superior-to-bin-bash/21613039

It seems to be more common to use the first approach. And we don't have a
specific reason to prefer the latter. Moreover, other scripts in this repo,
like 'make-release' and 'whitespacecleanup.sh' use the former too.

docs: always use dev_requirements.txt together with main version constraints in setup.py

pip isn't good at considering old constraints when installing or updating
packages later. Instead, install everything at once.

Note: pip and setuptools are usually installed automatically and could perhaps
be removed from documentation. But for now, let's keep it.

scripts: add developer script validate-commits

validate-commits can be used to verify a series of commits before
submitting/pushing it. It will create a virtualenv and run run-all-cleanup
and pytest.

Invocation:
validate-commits <revset>

Output:
OK: 99c28d4459286261f7e798ab3fe60be3b0be2e7e (run-all-cleanup)
OK: 99c28d4459286261f7e798ab3fe60be3b0be2e7e (pytest)
OK: 8a79857fb430eea80556af8d7e7c88810eee7f25 (run-all-cleanup)
NOK: 8a79857fb430eea80556af8d7e7c88810eee7f25 (pytest)
OK: 42d4c56ad79c4a492b8bc607aa070de32d9c6073 (run-all-cleanup)
NOK: 42d4c56ad79c4a492b8bc607aa070de32d9c6073 (pytest)