Make dict views behave like their unrestricted versions

[perrinjerome]

unlike the restricted versions, the unrestricted versions:

• are not iterators, they are views
• have a len
• are false when the mapping is empty, true otherwise
• are instances of collections.abc.MappingView

This change aligns the behavior of restricted versions with the behavior of unrestricted one, while keeping the "guard", which validates the access with the security manager.

During this refactoring, also change .items() to validate each keys and values, like .keys() and .values() do.

[perrinjerome]

What was most problematic for me when porting a python2 code base was constructs like:

d = {}
...
if len(d.keys()):
  ...

which was causing an error "SafeIter has no len" and

d = {}
...
if d.keys():
  ...

which was silently true when d is empty.

Most of the actual implementation is delegated to collections.abc, so the code is not so complex.

[perrinjerome]

There was something wrong with the first version of the patch, it is now ok.

[dataflake]

Looks fine, but then again I do not have any deep knowledge about the code.

[d-maurer]

Even though I now know how to access branches in foreign repositories (thanks to @navytux), I would prefer if you would make your future PRs directly in the target repository.

[d-maurer]

Maybe, you consider adding my comment suggestion.

[icemac]

I think appveyor breaks because this PR is created from a fork of the repository instead of a branch. Could you please recreate it on a branch inside the zopefoundation repository to see that also the windows version runs successfully.

(As a rule of thumb: Creating branches in the main repostory (instead of forks) is running smoother and is liked more by the zopefoundation maintainers.)

[perrinjerome]

Thank you @icemac I made a new pull request from zopefoundation with the same changes #148

[perrinjerome]

After some time the appveyor build was OK, so I merged, thank you everybody !

[perrinjerome]

After some time the appveyor build was OK, so I merged, thank you everybody !

actually, it was probably not "after some time", but after I pushed the same commit in zopefoundation/AccessControl