CI: Replace minimal-ubuntu job with ubuntu

The case of testing that pystencils does not depend on any optional dependencies is still fulfilled by the minimal_conda image.

Depends on https://i10git.cs.fau.de/pycodegen/pycodegen/merge_requests/9. Fixes #19 (closed).

doc/notebooks/01_tutorial_getting_started.ipynb

 ``` python
-import requests
-import imageio
-from io import BytesIO
-
-response = requests.get("https://www.python.org/static/img/python-logo.png")
-img = imageio.imread(BytesIO(response.content)).astype(np.double)
-img /= img.max()
-plt.imshow(img);
+try:
+    import requests
+    import imageio
+    from io import BytesIO
+
+    response = requests.get("https://www.python.org/static/img/python-logo.png")
+    img = imageio.imread(BytesIO(response.content)).astype(np.double)
+    img /= img.max()
+    plt.imshow(img);
+except ImportError:
+    print("No requests installed")
+    img = np.random.random((82, 290, 4))
 %% Cell type:code id: tags: