fix pip breaking due to setuptools/pip/cryptography problem

pip<6.1 + setuptools>=34 have a problem with packages that
try to update setuptools during installation, like cryptography.
See https://github.com/pypa/pip/issues/4253. The Ubuntu 14.04
package versions are pip 1.5.4 and setuptools 3.3. When we
install cryptography under those versions, it tries to update
setuptools to version 34, which became available about 10 days
ago, and then pip gets permanently broken with errors like
"ImportError: No module named 'packaging'".

The easiest work-around on systems that aren't already broken is
to upgrade pip and setuptools individually before we install any
package that tries to update setuptools.

Also try to detect a broken system and forcibly remove setuptools
first before trying to install/upgrade pip.

fixes #1080, fixes #1081, fixes #1086
see #1083
see https://discourse.mailinabox.email/t/error-with-pip-and-python/1880
see https://discourse.mailinabox.email/t/error-installing-mib/1875

setup/management.sh

@@ -4,7 +4,10 @@ source setup/functions.sh
 
 echo "Installing Mail-in-a-Box system management daemon..."
 
-# Install packages.
+# DEPENDENCIES
+
+# Install Python packages that are available from the Ubuntu
+# apt repository:
 # flask, yaml, dnspython, and dateutil are all for our Python 3 management daemon itself.
 # duplicity does backups. python-pip is so we can 'pip install boto' for Python 2, for duplicity, so it can do backups to AWS S3.
 apt_install python3-flask links duplicity libyaml-dev python3-dnspython python3-dateutil python-pip
@@ -12,6 +15,29 @@ apt_install python3-flask links duplicity libyaml-dev python3-dnspython python3-
 # These are required to pip install cryptography.
 apt_install build-essential libssl-dev libffi-dev python3-dev
 
+# pip<6.1 + setuptools>=34 have a problem with packages that
+# try to update setuptools during installation, like cryptography.
+# See https://github.com/pypa/pip/issues/4253. The Ubuntu 14.04
+# package versions are pip 1.5.4 and setuptools 3.3. When we
+# install cryptography under those versions, it tries to update
+# setuptools to version 34, which now creates the conflict, and
+# then pip gets permanently broken with errors like
+# "ImportError: No module named 'packaging'".
+#
+# Let's test for the error:
+if ! python3 -c "from pkg_resources import load_entry_point" 2&> /dev/null; then
+	# This system seems to be broken already.
+	echo "Fixing broken pip and setuptools..."
+	rm -rf /usr/local/lib/python3.4/dist-packages/{pkg_resources,setuptools}*
+	apt-get install --reinstall python3-setuptools python3-pip python3-pkg-resources
+fi
+#
+# The easiest work-around on systems that aren't already broken is
+# to upgrade pip (to >=9.0.1) and setuptools (to >=34.1) individually
+# before we install any package that tries to update setuptools.
+hide_output pip3 install --upgrade pip
+hide_output pip3 install --upgrade setuptools
+
 # Install other Python 3 packages used by the management daemon.
 # The first line is the packages that Josh maintains himself!
 # NOTE: email_validator is repeated in setup/questions.sh, so please keep the versions synced.
@@ -24,6 +50,8 @@ hide_output pip3 install --upgrade \
 # S3 api used in some regions, which breaks backups to those regions.  See #627, #653.
 hide_output pip install --upgrade boto
 
+# CONFIGURATION
+
 # Create a backup directory and a random key for encrypting backups.
 mkdir -p $STORAGE_ROOT/backup
 if [ ! -f $STORAGE_ROOT/backup/secret_key.txt ]; then