add basic pytest-mpl test and CI setup

[PiotrZakrzewski]

As discussed #12
Adding a basic CI. For now just one test, the rest will be added when the issue failing other example scripts is fixed.

I encountered an issue with running the test on pigeon script files containing repressor element. It fails on

Traceback (most recent call last):
  File "/mnt/c/Users/piotr/code/pigeon/.venv/bin/pigeon", line 8, in <module>
    sys.exit(main())
  File "/mnt/c/Users/piotr/code/pigeon/cmdline.py", line 40, in main
    parser.parseAndGenerateImage(data)
  File "/mnt/c/Users/piotr/code/pigeon/pigeon/Pigeon.py", line 47, in parseAndGenerateImage
    start, end = dr.renderDNA(axis, design, part_renderers, regs = arcs, reg_renderers=dr.std_reg_renderers(), plot_vector=pigeon_listener.has_vector, vector_label=pigeon_listener.vector_label)
  File "/mnt/c/Users/piotr/code/pigeon/pigeon/dnaplotlib.py", line 3184, in renderDNA
    arcstart = (reg['from_part']['start'] + reg['from_part']['end']) / 2
TypeError: string indices must be integers

I will create a seperate issue for it