Change Python minimum version 3.7 -> 3.8

pyproject.toml

@@ -4,7 +4,7 @@ description="Opencv implementations of the operators and tensor of aidge framewo
 dependencies = [
     "numpy>=1.21.6",
 ]
-requires-python = ">= 3.7"
+requires-python = ">= 3.8"
 dynamic = ["version"] # defined by pbr
 readme = "README.md"
 license = { file = "LICENSE" }