Rework the pad_inches test and reorganize the express API test files

[YuEfSaEDU]

Reworks the pad_inches test and splits the express API tests into per-endpoint files. Fixes #104.

The pad test is back as a pixel test on the K5-minus-edge image at figsize=(4,3): from the midpoint of each image edge it walks in and expects pad_inches * dpi background pixels before the drawing, for pad values 0.0, 0.1 and 0.5. On the right it starts a quarter of the way down, because the rightmost ink there is a vertical edge line that ends above the bottom vertices. With bordered vertices the run measures a pixel short, so the check allows two pixels of slack.

Measured on current master: 0/10/50 background pixels at 0.0/0.1/0.5 on all four sides, with the image exactly 400x300 at every pad.

Renames: test_draw_colors.py → test_express_api_draw.py (also takes in test_draw_text, since ascii() is on the way out per #94), test_planarity.py → test_express_api_is_planar.py, test_planarity_networkx.py → test_express_api_networkx.py. The write() and kuratowski_edges() tests moved out to their own files.

Suite is 50 tests now, up from 48 — the pad test runs through both draw() entry points. One stale comment in test_png_background_and_transparency (it still says the saved image comes out smaller than the figsize) predates the figsize fix; I left it alone.

[john-boyer-phd]

This looks all good, YUSUF. Thanks for helping to get the code all organized, and for the excellent test of pad_inches. It looks like something I did in draw() makes Python 3.10 a little unhappy, and your test is catching that. However, we need to change out 3.10 for 3.15 in the October timeframe anyway, so it's best to sunset 3.10 now so all tests will pass on all Pythons that will be supported by 1.1.0.