Add missing dependency in requirements-dev.txt

[jtmiclat]

Hi i cloned the repo and tried running the following to run the tests

pip install -e .
pip install -r requirements-dev.txt 

but when i was running the test i hit the following error

❯ pytest dask_geopandas/tests
ERROR collecting dask_geopandas/tests/test_geohash.py _______________________________________________________________________________
ImportError while importing test module '/Users/jtmiclat/personal/dask-geopandas/dask_geopandas/tests/test_geohash.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../.pyenv/versions/3.9.16/lib/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
dask_geopandas/tests/test_geohash.py:6: in <module>
    from pygeohash import encode
E   ModuleNotFoundError: No module named 'pygeohash' 

Baed on

dask-geopandas/continuous_integration/envs/39-no-optional-deps.yaml

Lines 13 to 20 in d3e15d1

	# test dependencies
	- pytest
	- pytest-cov
	- hilbertcurve
	- pygeohash
	- pip
	- pip:
	- pymorton

requirements-dev.txt was missing pygeohash. After adding it to the requirments-dev.txt the test ran correctly

[martinfleis]

Thanks!