run mailconfig.py's email address validator outside of the virtualenv during questions.sh

We don't have the virtualenv this early in setup.

Broken by 0088fb45.

Fixes #1326.

See https://discourse.mailinabox.email/t/that-is-not-a-valid-email-error-during-mailinabox-installation/2793.

management/mailconfig.py

 #!/usr/local/lib/mailinabox/env/bin/python
 
+# NOTE:
+# This script is run both using the system-wide Python 3
+# interpreter (/usr/bin/python3) as well as through the
+# virtualenv (/usr/local/lib/mailinabox/env). So only
+# import packages at the top level of this script that
+# are installed in *both* contexts. We use the system-wide
+# Python 3 in setup/questions.sh to validate the email
+# address entered by the user.
+
 import subprocess, shutil, os, sqlite3, re
 import utils
 from email_validator import validate_email as validate_email_, EmailNotValidError

setup/questions.sh

@@ -12,7 +12,9 @@ if [ -z "$NONINTERACTIVE" ]; then
 		apt_get_quiet install dialog python3 python3-pip  || exit 1
 	fi
 
-	# email_validator is repeated in setup/management.sh
+	# Installing email_validator is repeated in setup/management.sh, but in setup/management.sh
+	# we install it inside a virtualenv. In this script, we don't have the virtualenv yet
+	# so we install the python package globally.
 	hide_output pip3 install "email_validator>=1.0.0" || exit 1
 
 	message_box "Mail-in-a-Box Installation" \
@@ -49,7 +51,7 @@ you really want.
 			# user hit ESC/cancel
 			exit
 		fi
-		while ! management/mailconfig.py validate-email "$EMAIL_ADDR"
+		while ! python3 management/mailconfig.py validate-email "$EMAIL_ADDR"
 		do
 			input_box "Your Email Address" \
 				"That's not a valid email address.\n\nWhat email address are you setting this box up to manage?" \