install Python 3 packages in a virtualenv

The cryptography package has created all sorts of installation trouble over the last few years, probably because of mismatches between OS-installed packages and pip-installed packages. Using a virtualenv for all Python packages used by the management daemon should make sure everything is consistent.

See #1298, see #1264.

management/backup.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/local/lib/mailinabox/env/bin/python
 
 # This script performs a backup of all user data:
 # 1) System services are stopped.

management/daemon.py

@@ -1,5 +1,3 @@
-#!/usr/bin/python3
-
 import os, os.path, re, json, time
 import subprocess
 

management/dns_update.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/local/lib/mailinabox/env/bin/python
 
 # Creates DNS zone files for all of the domains of all of the mail users
 # and mail aliases and restarts nsd.

management/email_administrator.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/local/lib/mailinabox/env/bin/python
 
 # Reads in STDIN. If the stream is not empty, mail it to the system administrator.
 

management/mail_log.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/local/lib/mailinabox/env/bin/python
 import argparse
 import datetime
 import gzip

management/mailconfig.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/local/lib/mailinabox/env/bin/python
 
 import subprocess, shutil, os, sqlite3, re
 import utils

management/ssl_certificates.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/local/lib/mailinabox/env/bin/python
 # Utilities for installing and selecting SSL certificates.
 
 import os, os.path, re, shutil

management/status_checks.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/local/lib/mailinabox/env/bin/python
 #
 # Checks that the upstream DNS has been set correctly and that
 # TLS certificates have been signed, etc., and if not tells the user