Update setuptools pinned in pipstrap (#6699)

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.

letsencrypt-auto-source/letsencrypt-auto

@@ -1272,9 +1272,9 @@ PACKAGES = maybe_argparse + [
      'pip-{0}.tar.gz'.format(PIP_VERSION),
      '09f243e1a7b461f654c26a725fa373211bb7ff17a9300058b205c61658ca940d'),
     # This version of setuptools has only optional dependencies:
-    ('59/88/2f3990916931a5de6fa9706d6d75eb32ee8b78627bb2abaab7ed9e6d0622/'
-     'setuptools-29.0.1.tar.gz',
-     'b539118819a4857378398891fa5366e090690e46b3e41421a1e07d6e9fd8feb0'),
+    ('37/1b/b25507861991beeade31473868463dad0e58b1978c209de27384ae541b0b/'
+     'setuptools-40.6.3.zip',
+     '3b474dad69c49f0d2d86696b68105f3a6f195f7ab655af12ef9a9c326d2b08f8'),
     ('c9/1d/bd19e691fd4cfe908c76c429fe6e4436c9e83583c4414b54f6c85471954a/'
      'wheel-0.29.0.tar.gz',
      '1ebb8ad7e26b448e9caa4773d2357849bf80ff9e313964bcaf79cbf0201a1648')