Modernize Python 2 code to prepare for Python 3

[cclauss]

Fixes issues discovered by the flake8 tests running on Travis CI.

Use print() function in both Python 2 and Python 3

• Legacy print statements are syntax errors in Python 3 but print() function works as expected in both Python 2 and Python 3.

Old style exceptions --> new style for Python 3

• Old style exceptions are syntax errors in Python 3 but new style exceptions work as expected in both Python 2 and Python 3.

Octal literals must start with 0o in Python 3.

In requirements.txt the ldap module has been a dummy for a long time now. We substitute a newer module for access to MySQL.

In .travis.yml run the Python 3 tests in allow_failures mode until we can get web.py to be Python 3 compatible.

Please test these changes carefully before merging this PR especially in areas related to LDAP and MySQL.

[iredmail]

Thanks @cclauss. :)

• ldap module should be python-ldap.
• I use web.py-0.40.dev1 for testing, it partly works with py3.
• Disabled tests in .travis.yml since unit testing requires a working iRedMail server (with SQL/LDAP service running).

[iredmail]

The plan is making it working with py3 only in the future, no more code for py2+py3 compatible.