Add initial project structure and tests for Elytra PIM Client

- Created pyproject.toml for project metadata and dependencies.
- Added requirements.txt for development and production dependencies.
- Implemented basic test structure in the tests module.
- Developed unit tests for ElytraClient, including Pydantic validation and error handling.

pyproject.toml

@@ -0,0 +1,63 @@
+[build-system]
+requires = ["setuptools>=65.0", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "elytra-pim-client"
+version = "0.1.0"
+description = "A Pythonic client for the Elytra PIM API"
+readme = "README.md"
+requires-python = ">=3.9"
+license = {text = "MIT"}
+authors = [
+    {name = "Your Name", email = "your.email@example.com"}
+]
+keywords = ["elytra", "pim", "api", "client"]
+classifiers = [
+    "Development Status :: 3 - Alpha",
+    "Intended Audience :: Developers",
+    "License :: OSI Approved :: MIT License",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3.9",
+    "Programming Language :: Python :: 3.10",
+    "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
+]
+
+dependencies = [
+    "requests>=2.28.0",
+    "python-dotenv>=0.21.0",
+    "pydantic>=2.0.0",
+]
+
+[project.optional-dependencies]
+dev = [
+    "pytest>=7.0.0",
+    "pytest-cov>=4.0.0",
+    "black>=23.0.0",
+    "isort>=5.12.0",
+    "flake8>=6.0.0",
+    "mypy>=1.0.0",
+]
+
+[project.urls]
+Repository = "https://git.him-tools.de/HIM-public/elytra_client.git"
+Documentation = "https://www.elytra.ch/"
+Issues = "https://git.him-tools.de/HIM-public/elytra_client/issues"
+
+[tool.setuptools]
+packages = ["elytra_client"]
+
+[tool.black]
+line-length = 100
+target-version = ['py39', 'py310', 'py311', 'py312']
+
+[tool.isort]
+profile = "black"
+line_length = 100
+
+[tool.mypy]
+python_version = "3.9"
+warn_return_any = true
+warn_unused_configs = true
+disallow_untyped_defs = false