Fixes#6697.
This PR updates the version of setuptools pinned in pipstrap which works around the problems we have seen on recent OSes.
Why did I pick this version of setuptools? Because it's the latest and greatest, [supports all versions of Python that we do](https://github.com/pypa/setuptools/blob/v40.6.3/setup.py#L173), [has been out for a month and a half without the need for a point release](https://setuptools.readthedocs.io/en/latest/history.html), and has no non-optional dependencies.
For the last point about dependencies, I don't think we have too much to worry about. setuptools did have a period between versions 34.0.0 and 36.0.0 where they tried to have normal dependencies on other packages, but reverted these changes. See their [changelog for 36.0.0](https://setuptools.readthedocs.io/en/latest/history.html#v36-0-0).
You can also compare their [current setup.py file](https://github.com/pypa/setuptools/blob/v40.6.3/setup.py) to the [setup.py file for the currently pinned version](https://github.com/pypa/setuptools/blob/v29.0.1/setup.py) and you'll see [not much has changed](https://pastebin.com/nQj6d7D8).
Not only that, but I have successfully tested these changes on:
* ubuntu18.10
* ubuntu18.04LTS
* ubuntu16.04LTS
* ubuntu14.04LTS
* ubuntu14.04LTS_32bit
* debian9
* debian8.1
* amazonlinux-2015.09.1
* amazonlinux-2015.03.1
* RHEL7
* fedora23
* fedora29
* centos7
* centos6
* freebsd11
* macOS
* Update setuptools to 40.6.3.
* Build letsencrypt-auto.
* update changelog
* Don't use pipstrap in Dockerfile.centos6.
* If there's no python or there's only python2.6 on red hat systems, install python3
* Always check for python2.6
* address style, documentation, nits
* factor out all initialization code
* fix up python version return value when no python installed
* add no python error and exit
* document DeterminePythonVersion parameters
* build letsencrypt-auto
* close brace
* build leauto
* fix syntax errors
* set USE_PYTHON_3 for all cases
* rip out NOCRASH
* replace NOCRASH, update LE_PYTHON set logic
* use built-in venv for py3
* switch to LE_PYTHON not affecting bootstrap selection and not overwriting LE_PYTHON
* python3ify fetch.py
* get fetch.py working with python2 and 3
* don't verify server certificates in fetch.py HttpsGetter
* Use SSLContext and an environment variable so that our tests continue to never verify server certificates.
* typo
* build
* remove commented out code
* address review comments
* add documentation for YES_FLAG and QUIET_FLAG
* Add tests to centos6 Dockerfile to make sure we install python3 if and only if appropriate to do so.
* Use pipstrap to install a good version of pip
* Use pytest in cb-auto tests
* Remove nose usage in auto_test.py
* remove nose dev dep
* use pytest in test_tests
* Use pytest in tox
* Update dev dependency pinnings
* remove nose multiprocess lines
* Use pytest for coverage
* Use older py and pytest for old python versions
* Add test for Error.__str__
* pin pytest in oldest test
* Fix tests for DNS-DO plugin on py26
* Work around bug for Python 3.3
* Clarify dockerfile comments
