examples: make pytest import optional

[mloubout]

Fix #2454

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 87.05%. Comparing base (42398c4) to head (4698a7d).

Files with missing lines	Patch %	Lines
examples/seismic/acoustic/acoustic_example.py	50.00%	2 Missing :warning:
examples/seismic/elastic/elastic_example.py	50.00%	2 Missing :warning:
examples/seismic/model.py	50.00%	2 Missing :warning:
examples/seismic/self_adjoint/test_utils.py	50.00%	2 Missing :warning:
...amples/seismic/self_adjoint/test_wavesolver_iso.py	50.00%	2 Missing :warning:
examples/seismic/test_seismic_utils.py	50.00%	2 Missing :warning:
examples/seismic/tti/tti_example.py	50.00%	2 Missing :warning:
...les/seismic/viscoacoustic/viscoacoustic_example.py	50.00%	2 Missing :warning:
...mples/seismic/viscoelastic/viscoelastic_example.py	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2455 +/- ## ========================================== - Coverage 87.08% 87.05% -0.04% ========================================== Files 238 238 Lines 45118 45145 +27 Branches 8408 8408 ========================================== + Hits 39291 39300 +9 - Misses 5094 5112 +18 Partials 733 733 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[mloubout]

Dione in #2456