Fix TypeError for datetime dict keys with key-cleaning flags (fixes #550)

[gaoflow]

Summary

Using a datetime/date as a dict key together with one of the
key-cleaning flags raised a TypeError:

import datetime
from deepdiff import DeepDiff

dt = datetime.datetime(2020, 5, 17, 22, 15)
DeepDiff({dt: 10.0}, {dt: 10}, ignore_numeric_type_changes=True)
# TypeError: type datetime.datetime doesn't define __round__ method

The same happens with ignore_string_case and ignore_string_type_changes
(any flag that routes dict keys through _get_clean_to_keys_mapping), and with
datetime.date keys.

Root cause

_get_clean_to_keys_mapping cleans numeric keys via number_to_string, whose
type guard is:

if not isinstance(number, numbers):
    return number

numbers is defined as only_numbers + datetimes (datetimes are included
because they are orderable). So a datetime key passes the guard and falls
through to round(number, ndigits=significant_digits), which datetimes do not
implement → TypeError.

Fix

Guard on only_numbers instead. Datetimes/dates (and anything else that is not
a real, round()-able number) are returned unchanged, exactly like every other
non-numeric value — which is what the early-return branch is for. Numeric
behaviour (int/float/Decimal/Fraction/complex/numpy, significant-digit
rounding) is unchanged.

Verification

dt = datetime.datetime(2020, 5, 17, 22, 15)
DeepDiff({dt: 10.0}, {dt: 10}, ignore_numeric_type_changes=True)   # {}  (was TypeError)

Added test_datetime_dict_key_with_ignore_flags to
tests/test_diff_datetime.py. It fails on dev
(TypeError ... __round__) and passes with this change. The existing
tests/test_diff_datetime.py and tests/test_helper.py suites (168 tests)
still pass.

Fixes #550.