Close matplotlib figures after testing

[pavoljuhas]

Add test fixture that cleans up any open matplotlib figures and
plug it in the tests that open new figures.
Fix RuntimeWarning: More than 20 figures have been opened.

[pavoljuhas]

For the record the following patch can be used to find tests that leave opened figures.

diff --git a/conftest.py b/conftest.py
index e74e741e..34bc013f 100644
--- a/conftest.py
+++ b/conftest.py
@@ -28,6 +28,14 @@ def pytest_configure(config):
         config.addinivalue_line("filterwarnings", f"ignore:Call to deprecated create function {f}")
 
 
+import pytest
+@pytest.fixture(autouse=True)
+def failonleftoverfigures(request):
+    import matplotlib.pyplot as plt
+    yield
+    assert not plt.get_fignums()
+
+
 def pytest_addoption(parser):
     parser.addoption(
         "--rigetti-integration",

[MichaelBroughton]

LGTM

[CirqBot]

Automerge cancelled: There are merge conflicts.