Add Tox

Add Tox as infrastructure to run various Python checks. For now,
add pytest to run all DRC verification tests.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>

requirements-test.txt

@@ -0,0 +1,4 @@
+iniconfig==2.0.0
+packaging==24.2
+pluggy==1.5.0
+pytest==8.3.3

tox.ini

@@ -0,0 +1,19 @@
+[tox]
+envlist = py312-test
+minversion = v4.0
+
+[testenv:{py39,py310,py311,py312}-test]
+basepython =
+  py39: python3.9
+  py310: python3.10
+  py311: python3.11
+  py312: python3.12
+setenv =
+  py39: PREFIX = py39-
+  py310: PREFIX = py310-
+  py311: PREFIX = py311-
+  py312: PREFIX = py312-
+deps =
+  test: -rrequirements-test.txt
+commands =
+  test: pytest ihp-sg13g2/libs.tech/klayout/tech/drc/tests