This issue is the equivalent of issue #2280 (and subsequent PR #2285) for the el_GR locale. Specifically:
Even though the documentation (v40.1.0) claims that a bank name can be generated by calling the bank method on the el_GR locale. As demonstrated below, this is false:

>>> import faker
>>> fake = faker.Faker(locale="el_GR")
>>> fake.bank()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "(...)\faker\faker\providers\bank\__init__.py", line 54, in bank
    raise AttributeError(
AttributeError: The Provider provider does not have a 'banks' attribute. Consider contributing to the project and  adding a 'banks' tuple to enable bank name generation.

I am willing to start working on this issue right away. I could populate the missing banks tuple in faker/providers/bank/el_GR/__init__.py with a list of active Greek financial institutions (Systemic and Cooperative banks).