Fix DeepSource alerts

• Unguarded next inside generator

  • Calls to next() should be inside try-except block.

    When the iterator is exhausted, StopIteration exception is raised. When used inside a generator, this can cause unexpected behavior. If not handled, it will propagate out of the generator causing termination. PEP-479 has been accepted to fix this problem. It will modify the behavior of StopIteration in generators.

    Each call to next() should be wrapped in a try-except block to explicitly handle StopIteration exceptions.

The existing code was mostly correct, this can be seen as a false positive. However, the new code is arguably more Pythonic and more efficient because it iterates only once. Also, there was a small inconsistency in test-fortinet-login.py, where this line tests c.value:

if any(c.name == 'SVPNCOOKIE' and c.value for c in s.cookies):

while this one doesn't:

    cookie = next(c.value for c in s.cookies if c.name == 'SVPNCOOKIE')

Partially fixes #341 (closed).