Z3c-authviewlet

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- Add support for Python 3.8 and 3.9.

- Drop support for Python 3.4.

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- Add support for Python 3.7.

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- Fix dependencies declared in `setup.py`.

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- Update to Python 3. Now supporting: Python 3.4 to 3.6, PyPy2 and PyPy3.

- Update the tests to `zope.testbrowser >= 5.0`.

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- Adapted test set up to the changes in `z3c.layer.pagelet` 1.9 thus
requiring at least this version now.

- Moved code from page template of session credentials login page to view
class so it can be customized. (Taken from
`zope.app.authentication.browser.loginform.LoginForm`.) Moved view class
``SessionCredentialsLoginForm`` from `z3c.authviewlet.auth` to
`z3c.authviewlet.session`.

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- Moved files in `z3c.authviewlet.browser` to `z3c.authviewlet` as
we only have browser code in this package.

- Broke dependency on `zope.app.publisher` by defining our own
``ILogin`` interface.