style: add flake8, black and isort

Author: impocode

.flake8

@@ -0,0 +1,3 @@
+[flake8]
+max-line-length = 100
+exclude = .git,__pycache__

Makefile

@@ -0,0 +1,4 @@
+style:
+	black .
+	isort .
+	flake8 .

pyproject.toml

@@ -12,7 +12,8 @@ readme = "readme.md"
 keywords = ["jsonapi", "pydantic"]
 classifiers = [
     "License :: OSI Approved :: MIT License",
-    "Programming Language :: Python :: 3.7",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3 :: Only",
     "Programming Language :: Python :: 3.8",
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
@@ -25,19 +26,24 @@ repository = "https://github.com/impocode/jsonapi-pydantic"
 documentation = "https://github.com/impocode/jsonapi-pydantic"
 
 [tool.poetry.dependencies]
-python = "^3.7"
+python = "^3.8.1"
 pydantic = "^2.1.1"
 
 [tool.poetry.urls]
 "Bug Tracker" = "https://github.com/impocode/jsonapi-pydantic/issues"
 
+[tool.poetry.group.dev.dependencies]
+black = "^23.12.0"
+isort = "^5.13.2"
+flake8 = "^6.1.0"
+
 [tool.isort]
-line_length = 99
+line_length = 100
 multi_line_output = 3
 include_trailing_comma = false
 
 [tool.black]
-line-length = 99
+line-length = 100
 
 [build-system]
 requires = ["poetry-core>=1.0.0"]