CI tests failing due to linking error in conda-based pytest #735

from scipy.signal import tf2zpk, zpk2tf, cont2discrete
[17](https://github.com/python-control/python-control/runs/6637792716#step:5:18)
/usr/share/miniconda/envs/test-environment/lib/python3.9/site-packages/scipy/signal/__init__.py:309: in <module>
[18](https://github.com/python-control/python-control/runs/6637792716#step:5:19)
    from . import _sigtools, windows
[19](https://github.com/python-control/python-control/runs/6637792716#step:5:20)
/usr/share/miniconda/envs/test-environment/lib/python3.9/site-packages/scipy/signal/windows/__init__.py:41: in <module>
[20](https://github.com/python-control/python-control/runs/6637792716#step:5:21)
    from ._windows import *
[21](https://github.com/python-control/python-control/runs/6637792716#step:5:22)
/usr/share/miniconda/envs/test-environment/lib/python3.9/site-packages/scipy/signal/windows/_windows.py:7: in <module>
[22](https://github.com/python-control/python-control/runs/6637792716#step:5:23)
    from scipy import linalg, special, fft as sp_fft
[23](https://github.com/python-control/python-control/runs/6637792716#step:5:24)
/usr/share/miniconda/envs/test-environment/lib/python3.9/site-packages/scipy/fft/__init__.py:91: in <module>
[24](https://github.com/python-control/python-control/runs/6637792716#step:5:25)
    from ._helper import next_fast_len
[25](https://github.com/python-control/python-control/runs/6637792716#step:5:26)
/usr/share/miniconda/envs/test-environment/lib/python3.9/site-packages/scipy/fft/_helper.py:3: in <module>
[26](https://github.com/python-control/python-control/runs/6637792716#step:5:27)
    from ._pocketfft import helper as _helper
[27](https://github.com/python-control/python-control/runs/6637792716#step:5:28)
/usr/share/miniconda/envs/test-environment/lib/python3.9/site-packages/scipy/fft/_pocketfft/__init__.py:3: in <module>
[28](https://github.com/python-control/python-control/runs/6637792716#step:5:29)
    from .basic import *
[29](https://github.com/python-control/python-control/runs/6637792716#step:5:30)
/usr/share/miniconda/envs/test-environment/lib/python3.9/site-packages/scipy/fft/_pocketfft/basic.py:6: in <module>
[30](https://github.com/python-control/python-control/runs/6637792716#step:5:31)
    from . import pypocketfft as pfft
[31](https://github.com/python-control/python-control/runs/6637792716#step:5:32)
E   ImportError: /usr/share/miniconda/envs/test-environment/lib/python3.9/site-packages/scipy/linalg/../../../../libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /usr/share/miniconda/envs/test-environment/lib/python3.9/site-packages/scipy/fft/_pocketfft/pypocketfft.cpython-39-x86_64-linux-gnu.so)