fix missing Path3DCollection._z_markers_idx attribute

The issue is in Matplotlib. This is a temporary solution before [1] is merged.

Fixes #407.

[1] https://github.com/matplotlib/matplotlib/pull/20416
27 jobs for master in 48 minutes and 22 seconds (queued for 9 seconds)
Status Name Job ID Coverage
  Build Env
manual build-env:conda #207794
allowed to fail manual
manual build-env:debian #207791
allowed to fail manual
manual build-env:default #207796
allowed to fail manual
manual build-env:ubuntu #207789
allowed to fail manual
 
  Build
passed build:bleeding-edge #207807

00:13:48

passed build:debian #207800

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passed build:latest #207805

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passed build:no-extras #207803

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passed build:stable #207802

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passed build:ubuntu #207798

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  Test
passed build PDF documentation #207829

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passed build documentation #207827

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passed check for broken links in doc #207831

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passed check for dependencies installed #207811

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passed check whitespace style #207809

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passed coverage:latest #207825

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failed test:bleeding-edge #207823
allowed to fail

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passed test:debian #207819

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passed test:latest #207821

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passed test:no-extras #207815

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passed test:stable #207813

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passed test:ubuntu #207817

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  Deploy
manual remove_coverage #207834
allowed to fail manual
manual remove_docs #207838
allowed to fail manual
passed upload coverage #207833

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passed upload dev version docs #207840

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passed upload documentation to the test server #207836

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Name Stage Failure
failed
test:bleeding-edge Test
1.83s call     kwant/tests/test_plotter.py::test_spectrum[matplotlib]
1.65s call kwant/physics/tests/test_gauge.py::test_minimal_cycle_basis[2-honeycomb-half-ring]
1.40s call kwant/tests/test_wraparound.py::test_plot_2d_bands
============ 2 failed, 548 passed, 21 warnings in 79.10s (0:01:19) =============
Authenticating with credentials from /root/.docker/config.json
Uploading artifacts...
tests.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=207823 responseStatus=201 Created token=ZSeeoLyG
ERROR: Job failed: exit code 1