Remove default_value_type inference from TraitType subclasses

I've created a series of PRs aimed at bypassing the default_value_type inference code for `TraitType` subclasses. See #1531, #1532, #1536.

Code link: https://github.com/enthought/traits/blob/9f934334668c6c0bf1e2866d6a81417275bbc255/traits/trait_type.py#L34-L52

Rationale:

- making Traits less DWIMmy; in general we want to make the Traits codebase more predictable and reduce the number of corner cases with potentially surprising behaviour
- code complexity: if we can remove the `_infer_default_value_type` function completely then some circular import problems go away - the dependence of that function on `TraitListObject`, `TraitDictObject` and `TraitSetObject` in particular is problematic.
- performance (minor): in the vast majority of cases we go through the inference code even when it's clear from context that the `default_value_type` should be constant. By setting `default_value_type` up front we avoid that code path.

Steps:

- [ ] Make sure that none of the built-in TraitType subclasses are using the default value type inference
- [x] Deal with obviously constant defaults: #1531, and a few more in #1539
- [x] Deal with `Any`: #1532 (this is the only case where list and dict default types are of interest)
- [x] Deal with `BaseClass` and its subclasses: #1536 (some care required in `BaseInstance`)
- [x] Deal with `Constant`: #1540
- [x] Deal with `TraitType` subclasses in tests: #1539
- [x] Determine whether `Symbol` needs changes: it doesn't - Symbol is deprecated in #1542
- [x] Deal with `Event` and `Disallow` (related: #1546)
- [x] Deal with `Delegate` and its subclasses.
- [ ] Loudly deprecate the use of default value type inference for external TraitType subclasses
- [ ] In Traits 7.0, replace `default_value_type = DefaultValue.unspecified` with `default_value_type = DefaultValue.constant` in the `TraitType` base class, and clean up the other classes accordingly.