ci: add config for black+isort

As recommended for existing projects, string normalization is switched
off:
https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#strings

Other config options, such as line width, are explained here:
https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#configuration-format

A config for isort was added in to ensure that isort is applied
in agreement with black once/if isort is applied

pyproject.toml

@@ -0,0 +1,14 @@
+[tool.black]
+skip-string-normalization = true
+target-version = [
+    "py36",
+    "py37",
+    "py38",
+    "py39",
+]
+
+[tool.isort]
+profile = "black"
+src_paths = [
+    "sympy",
+]