TST: suppress jinja2 deprecation warning

[rgommers]

This is triggered by Sphinx, which is imported during test discovery for
doc/sphinxext/numpydoc/tests/test_numpydoc.py,
and breaks running the test suite in a development branch.

Relevant part of the traceback:

____________ ERROR collecting doc/sphinxext/numpydoc/tests/test_numpydoc.py ____________
../scipy/doc/sphinxext/numpydoc/tests/test_numpydoc.py:3: in <module>
    from numpydoc.numpydoc import mangle_docstrings, _clean_text_signature

...

        __name__   = 'sphinx.jinja2glue'
        __package__ = 'sphinx'
        __spec__   = ModuleSpec(name='sphinx.jinja2glue', loader=<_frozen_importlib_external.SourceFileLoader object at 0x7f014be80070>, origin='/home/rgommers/anaconda3/envs/scipy-dev/lib/python3.8/site-packages/sphinx/jinja2glue.py')
        __warningregistry__ = {'version': 126}
        _slice_index = <function _slice_index at 0x7f014c001d30>
        _tobool    = <function _tobool at 0x7f014c001430>
        _todim     = <function _todim at 0x7f014c001ca0>
        _toint     = <function _toint at 0x7f014c001c10>
        accesskey  = <function accesskey at 0x7f014c001dc0>
        contextfunction = <function contextfunction at 0x7f014cfb8160>
        idgen      = <class 'sphinx.jinja2glue.idgen'>
        logging    = <module 'sphinx.util.logging' from '/home/rgommers/anaconda3/envs/scipy-dev/lib/python3.8/site-packages/sphinx/util/logging.py'>
        mtimes_of_files = <function mtimes_of_files at 0x7f014c9d28b0>
        open_if_exists = <function open_if_exists at 0x7f014cfb8670>
        path       = <module 'posixpath' from '/home/rgommers/anaconda3/envs/scipy-dev/lib/python3.8/posixpath.py'>
        pformat    = <function pformat at 0x7f015b839940>
../../anaconda3/envs/scipy-dev/lib/python3.8/site-packages/jinja2/utils.py:110: in contextfunction
    warnings.warn(
E   DeprecationWarning: 'contextfunction' is renamed to 'pass_context', the old name will be removed in Jinja 3.1.

EDIT: relevant package versions:

jinja2                    3.0.1              pyhd8ed1ab_0    conda-forge
numpydoc                  1.1.0                      py_1    conda-forge
sphinx                    3.5.4              pyh44b312d_0    conda-forge

[tylerjereddy]

LGTM, I note another unrelated CI failure popping up (one of them below the fold). @mdhaber have you seen that one sporadically (I haven't had a chance to scan all recent issues yet)

________________ TestStudentizedRange.test_moment_vectorization ________________
[gw0] linux -- Python 3.8.0 /usr/bin/python3.8
scipy/stats/tests/test_distributions.py:4628: in test_moment_vectorization
    m = stats.studentized_range._munp([1, 2], [4, 5], [10, 11])
        self       = <scipy.stats.tests.test_distributions.TestStudentizedRange object at 0xd1874d00>
scipy/stats/_continuous_distns.py:9495: in _munp
    return np.float64(ufunc(K, k, df))
E   RuntimeWarning: invalid value encountered in ? (vectorized)
        K          = [1, 2]
        _a         = 0
        _b         = inf
        _single_moment = <function studentized_range_gen._munp.<locals>._single_moment at 0xd1853fa0>
        cython_symbol = '_studentized_range_moment'
        df         = [10, 11]
        k          = [4, 5]
        self       = <scipy.stats._continuous_distns.studentized_range_gen object at 0xeba21520>
        ufunc      = <ufunc '? (vectorized)'>
- generated xml file: /scipy/build/testenv/lib/python3.8/site-packages/junit/test-results.xml -