implements MasterKeyChangeForce Meta key

[KeyWeeUsr]

This PR implements the forceful password change trigger which happens once a person opens the DB in a KDBX-compatible UI tool.

• The key name is defined here and is always present in the DB XML.
• The type is always C# Long
• The behavior is triggered after >= 0 check

Closes #269

[KeyWeeUsr]

Simple example:

from pykeepass import create_database

db = create_database("db.kdbx")
assert db.password_change_force_days == -1
assert not db.needs_password_change
print(db.xml())

# open with the password for blank DB (embedded in the codebase) via UI tool
# no effect, close

# enforce password change
db.password_change_force_days = 0  # immediately
assert db.password_change_force_days == 0
assert db.needs_password_change
db.save()
print(db.xml())

# open with the password for blank DB (embedded in the codebase) via UI tool
# UI needs to enforce master password change immediately

[pschmitt]

lgtm

[KeyWeeUsr]

@pschmitt Please approve it via Review changes if it's okay or add hacktoberfest-accepted label for it. Thanks! :)

[Evidlo]

We need to check when the masterkey was set for this to make sense. I don't see any datetime parsing happening here, so I don't understand how this is supposed to work.

These are the fields we need to read/set:

    <MasterKeyChanged>2017-06-17T00:38:50Z</MasterKeyChanged>
    <MasterKeyChangeRec>-1</MasterKeyChangeRec>
    <MasterKeyChangeForce>100</MasterKeyChangeForce>

So something like:

@property
def masterkey_change_required(self):
    change_date = self._decode_time(self._xpath('/KeePassFile/Meta/MasterKeyChanged'))
    max_days = self._xpath('/KeePassFile/Meta/MasterKeyChangeForce')
    return (datetime.now() - change_date).days > max_days

@property
def masterkey_change_recommended(self):
    change_date = self._decode_time(self._xpath('/KeePassFile/Meta/MasterKeyChanged'))
    max_days = self._xpath('/KeePassFile/Meta/MasterKeyChangeRec')
    return (datetime.now() - change_date).days > max_days

Also _decode_time should be moved out of BaseElement into PyKeePass.

And tests.

[Evidlo]

Looking at the keepass2 code linked above, I think the booleans bChangeRec and bChangeForce are just to determine whether the checkboxes in the settings menu are ticked or not when a database is loaded:

[Evidlo]

This was implemented in 9811ac0