Fix naturaldelta doctest

[hugovk]

Before

❯ pytest --doctest-modules src/humanize/time.py -q
...F.                                                                         [100%]
===================================== FAILURES ======================================
_______________________ [doctest] humanize.time.naturaldelta ________________________
125
126     ```pycon
127     >>> import datetime as dt
128     >>> from dateutil.tz import gettz
129
130     >>> berlin = gettz("Europe/Berlin")
131     >>> now = dt.datetime.now(tz=berlin)
132     >>> later = now + dt.timedelta(minutes=30)
133
134     >>> assert naturaldelta(later - now) == "30 minutes"
Expected:
    True
    ```
Got nothing

/Users/hugo/github/humanize/src/humanize/time.py:134: DocTestFailure
============================== short test summary info ==============================
FAILED src/humanize/time.py::humanize.time.naturaldelta
1 failed, 4 passed in 0.04s

After

❯ pytest --doctest-modules src/humanize/time.py -q
.....                                                                         [100%]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.69%. Comparing base (ffdf407) to head (3829f92).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #402   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files          12       12           
  Lines         983      983           
=======================================
  Hits          980      980           
  Misses          3        3           

Flag	Coverage Δ
macos-latest	97.55% <ø> (ø)
ubuntu-latest	97.55% <ø> (ø)
windows-latest	93.18% <ø> (ø)

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