fix: make Delorean hashable by defining __hash__

Defining __eq__ without __hash__ sets __hash__ to None in Python 3, so
Delorean instances were unhashable and could not be used in sets or as
dict keys (unlike datetime). Hash on self.epoch, the same value __eq__
compares, so equal instances hash equal.

PR.md

@@ -0,0 +1,65 @@
+# 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'`.