Refactor internal scope handling by introducing Scope enum

This replaces the internal use of string literals and "scopenum"
to a proper Scope enum, which also centralizes indexes, getting
higher scopes, get next scope in the order, etc. Another benefit
is that it helps the type checker by introducing a proper type,
and improve readability (IMHO).

This is _mostly_ an internal change, however due to historical
reasons, the API of the following _internal_ objects has
changed slightly:

* `CallSpec2`: `_arg2scopenum` renamed to `_arg2scope` for consistency.
* `FixtureRequest`:
  Previously contained a `scope: str` attribute.
  Changed attribute to `_scope: Scope`, with a backward compatible read-only property `scope -> str`.
  This one might cause some confusion, because the internal attribute has a different type
  than the public property of the same name.
* `SubRequest.__init__` parameter changed from `_Scope` to `Scope`.

Author: nicoddemus