traitlets Dict should support a strict keys mode

Please add support for a way to restrict keys on a `Dict`...

Example of what should raise a `TraitError`

```python
from traitlets import HasTraits, Dict, Unicode

class Foo(HasTraits):
# the strict_keys keyword is a suggested feature...
data = Dict(per_key_traits={"this_key": Unicode()}, strict_keys=True)

foo = Foo()

# This is fine...
foo.data = {"this_key": "value"}

# This should raise a TraitError
foo.data = {"invalid_key": None}
```

Start by locating the Dict implementation and its handling of per_key_traits. Add coverage for the strict_keys example, including acceptance of declared keys and a TraitError for invalid keys; done means the requested behavior is tested and works.

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