Migrate certbot-auto users on CentOS 6 to Python 3.6 (#7268)

Fixes #7007

Python 3.4 is [EOL](https://www.python.org/dev/peps/pep-0429/), and only Python 3.x version available for CentOS 6 through EPEL is this version, and so is used by `certbot-auto`, the only official way to install Certbot on this platform.

This unpleasant situation becomes a little more uncomfortable, considering that the newest `pip` version (19.2) [just dropped Python 3.4 support](https://github.com/pypa/pip/issues/6685) and will refuse to start on this Python version. We can expect a lot of dependencies to follow this path now.

One direct result of this situation is that a fix to support correctly the ARM platforms requires to upgrade `pip` to 19.2 for `certbot-auto`. So this is not possible right now.

Then, let's upgrade Certbot instances on CentOS 6 to a supported version of Python 3.

This PR proposes a new bootstrap approach for CentOS 6 platform, `BootstrapRpmPython3Legacy`, that will install Python 3.6 from [SCL](https://www.softwarecollections.org) (the latest one available for now on CentOS 6). In term of Python 3 specific bootstrap methods, I take the occasion here to completely separate the bootstrap of CentOS 6 as a legacy system, from the RPM-based newest systems (like Fedora 29+) that are simply dropping support for Python 2.x. This is in prevision of future migration for all systems on Python 3.x, that is a different problematic than supporting old systems.

* Add logic

* Rebuilt letsencrypt-auto

* Fix logic

* Focus on specific packages

* Maintain PATH for further invocations of letsencrypt-auto after bootstrap.

* Various corrections

* Fix farm test for RHEL6

* Working centos6 letsencrypt-auto self tests

* Fix test_sdist for CentOS 6

* Corrections

* Work in progress

* Working configuration

* Fix typo

* Remove EPEL. Add a test.

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Improvements after review

* Improvements

* Add a comment

* Add a test

* Update a test

* Corrections

* Update function return

* Work in progress

* Correct behavior on oracle linux 6.

* Corrections

* Rebuild script

* Add letsencrypt-auto tests for oraclelinux6

* Update tox.ini

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/tests/oraclelinux6_tests.sh

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/tests/oraclelinux6_tests.sh

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Remove specific code for scientific linux

* Change some variables names

* Update letsencrypt-auto-source/tests/oraclelinux6_tests.sh

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Various corrections

* Fix tests

* Add a comment

* Update message

* Fix test message

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update scripts

* More focused assertion

* Add back a test

* Update script

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Update letsencrypt-auto-source/letsencrypt-auto.template

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

* Check quiet mode

* Add changelog

* Update letsencrypt-auto-source/tests/oraclelinux6_tests.sh

Co-Authored-By: Brad Warren <bmw@users.noreply.github.com>

tests/letstest/scripts/test_leauto_upgrades.sh

@@ -117,6 +117,8 @@ if ! diff letsencrypt-auto letsencrypt-auto-source/letsencrypt-auto ; then
 fi
 
 if [ "$RUN_RHEL6_TESTS" = 1 ]; then
+    # Add the SCL python release to PATH in order to resolve python3 command
+    PATH="/opt/rh/rh-python36/root/usr/bin:$PATH"
     if ! command -v python3; then
         echo "Python3 wasn't properly installed"
         exit 1

tests/letstest/scripts/test_sdists.sh

@@ -1,8 +1,21 @@
 #!/bin/sh -xe
 
 cd letsencrypt
+
+# If we're on a RHEL 6 based system, we can be confident Python is already
+# installed because the package manager is written in Python.
+if command -v python && [ $(python -V 2>&1 | cut -d" " -f 2 | cut -d. -f1,2 | sed 's/\.//') -eq 26 ]; then
+    # RHEL/CentOS 6 will need a special treatment, so we need to detect that environment
+    RUN_RHEL6_TESTS=1
+fi
+
 letsencrypt-auto-source/letsencrypt-auto --install-only -n --debug
 
+if [ "$RUN_RHEL6_TESTS" = 1 ]; then
+  # Enable the SCL Python 3.6 installed by letsencrypt-auto bootstrap
+  PATH="/opt/rh/rh-python36/root/usr/bin:$PATH"
+fi
+
 PLUGINS="certbot-apache certbot-nginx"
 PYTHON_MAJOR_VERSION=$(/opt/eff.org/certbot/venv/bin/python --version 2>&1 | cut -d" " -f 2 | cut -d. -f1)
 TEMP_DIR=$(mktemp -d)