* Fix vulnerability check in tc7200_password_disclosure_v2

Use an encrypted zero block to identify the binary settings backup data.

* Make tc7200_password_disclosure_v2 compatible with new pycrypto

The decryption routine uses AES in ECB mode. The pycrypto module is used for this routine. ECB does not involve any initialization vector (IV). However routersploit uses one that is 16 NUL bytes. Older pycrypto versions just ignored this, but newer versions (at least since 2.6) error on using an IV with ECB.

* tc7200_password_disclosure_v2: adapt test to the binary check

* add missing import

* Bump v3.0.0

* Adding Slack invitation link

Introduce `Exploit.validate_setup` method in order to check whether
Exploit's setup is correct. In most case scanarios we will check
if `Exploit.target` is not `None`. When exploit need custom
validation logic please overwrite `validate_setup`.

LGTM

* Move tests to tests/ directory
* Remove unused `tox.ini` file
* Add flake8 compliance
* Add `lint` target to `tests` target in the Makefile