# Make `Delorean` hashable again

## Problem

`Delorean` instances cannot be put in a `set` or used as `dict` keys:

```python
>>> from datetime import datetime
>>> from delorean import Delorean
>>> d = Delorean(datetime(2015, 1, 1), timezone="UTC")
>>> hash(d)
Traceback (most recent call last):
    ...
TypeError: unhashable type: 'Delorean'
>>> {d}
TypeError: unhashable type: 'Delorean'
```

This is surprising for a date/time value object: the standard library's
`datetime.datetime` is hashable, and `Delorean` supports equality and ordering,
so grouping instances in sets or keying dictionaries by them is a natural thing
to want.

## Root cause

`delorean/dates.py` defines `__eq__` (and the rich-comparison operators) on
`Delorean`, but does not define `__hash__`. In Python 3, defining `__eq__`
without `__hash__` sets `__hash__` to `None`, which makes the class unhashable:

```python
>>> Delorean.__hash__ is None
True
```

Under Python 2 (the version delorean originally targeted) defining `__eq__`
left the inherited `__hash__` in place, so this is effectively a regression
introduced by the move to Python 3.

## Fix

Add a `__hash__` consistent with the existing `__eq__`. `__eq__` compares
`self.epoch == other.epoch`, so hashing on `self.epoch` guarantees the hash
invariant (equal objects hash equal), including two instances describing the
same instant in different timezones:

```python
def __hash__(self):
    return hash(self.epoch)
```

## Test

Added two tests to `tests/delorean_tests.py`:

- `test_hashable`: `hash()` works and an instance can be stored in a set and
  used as a dict key.
- `test_hash_consistent_with_equality`: two equal instances (same instant,
  different timezone) hash equal and collapse to one element in a set.

## Gate

- `python -m pytest tests/` -> 232 passed, 44 subtests passed.

Counterfactual (fix reverted, tests kept): both new tests fail with
`TypeError: unhashable type: 'Delorean'`.