`Interval`'s type parameter no longer includes `None`. The exported type
variable `LessThanComparableOrNoneT` (bound to
`LessThanComparable | None`) was deprecated and replaced with
`LessThanComparableT` (bound to `LessThanComparable`).

`None` is still accepted as a value for `start` / `end` to indicate an
unbounded side, but it is treated purely as bound metadata rather than
a value in the interval's comparable space.

Note that explicitly using `Interval[int | None]` still works, as
`(int | None) | None` is equivalent to `int | None`, even when it is no
longer needed nor recommended. Old code will mostly work, but there is
one soft breaking change: `None in some_interval` and
`None in some_interval_set` is now a type-check error at call sites,
matching the design intent that `None` is a bound marker and never a
member value.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Add IntervalSet[T] to frequenz.core.math: a normalized, immutable set of
Interval[T] values that supports O(log n) membership testing via the `in`
operator.

On construction, IntervalSet sorts the passed intervals by start and
merges any that overlap or share an inclusive endpoint. Only the
normalized form is stored, so equality, hashing, and iteration are
well-defined regardless of input order; callers that need the original
inputs can keep them separately.

Unbounded intervals are handled via the existing Interval[T] convention:
a None start is treated as -infinity and a None end as +infinity during
sort/merge, so for example IntervalSet((Interval(None, 5), Interval(3,
None))) collapses to a single Interval(None, None).

The generic container-of-containers variant discussed during design is
deferred: there is no immediate use case, and adding IntervalSet alone
is a fully additive step that does not foreclose a future generic type
in a separate module.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>