refactor: migrate package management from poetry to pep 621 standard

Migrate package metadata and dependency configuration from Poetry to
PEP 621 standard using flit_core backend. Update documentation with
production and development setup instructions using venv, pip, or uv.

- update pyproject.toml to PEP 621 format with flit_core build-system
- bump minimum dependency versions to latest PyPI releases
- revise README.md with production and development setup guides

pyproject.toml

@@ -1,11 +1,21 @@
-[tool.poetry]
+[build-system]
+requires = ["flit_core>=3.12.0,<4.0.0"]
+build-backend = "flit_core.buildapi"
+
+[tool.flit.module]
+name = "mdrsclient"
+
+[project]
 name = "mdrs-client-python"
 version = "1.3.18"
 description = "The mdrs-client-python is python library and a command-line client for up- and downloading files to and from MDRS based repository."
-authors = ["Yoshihiro OKUMURA <yoshihiro.okumura@riken.jp>"]
-license = "MIT"
+authors = [
+    { name = "Yoshihiro OKUMURA", email = "yoshihiro.okumura@riken.jp" }
+]
+license = { text = "MIT" }
 readme = "README.md"
-classifiers=[
+requires-python = ">=3.10"
+classifiers = [
     "Development Status :: 4 - Beta",
     "Environment :: Console",
     "Intended Audience :: Developers",
@@ -18,33 +28,28 @@ classifiers=[
     "OSI Approved :: MIT License",
     "Topic :: Utilities",
 ]
-packages = [
-    { include = "mdrsclient" }
+
+dependencies = [
+    "requests>=2.34.2",
+    "requests-toolbelt>=1.0.0",
+    "python-dotenv>=1.2.2",
+    "pydantic>=2.13.4",
+    "pydantic-settings>=2.14.2",
+    "PyJWT>=2.13.0",
+    "validators>=0.35.0",
 ]
 
-[tool.poetry.dependencies]
-python = "^3.10"
-requests = "^2.34.2"
-requests-toolbelt = "^1.0.0"
-python-dotenv = "^1.1.0"
-pydantic = "^2.13.4"
-pydantic-settings = "^2.14.2"
-PyJWT = "^2.13.0"
-validators = "^0.35.0"
+[project.optional-dependencies]
+dev = [
+    "black>=26.5.1",
+    "flake8>=7.3.0",
+    "Flake8-pyproject>=1.2.4",
+    "isort>=8.0.1",
+    "pyright>=1.1.411",
+]
 
-[tool.poetry.group.dev.dependencies]
-black = "^26.5.1"
-flake8 = "^7.2.0"
-Flake8-pyproject = "^1.2.3"
-isort = "^8.0.1"
-pyright = "^1.1.411"
-
-[tool.poetry.scripts]
-mdrs = 'mdrsclient.__main__:main'
-
-[build-system]
-requires = ["poetry-core"]
-build-backend = "poetry.core.masonry.api"
+[project.scripts]
+mdrs = "mdrsclient.__main__:main"
 
 [tool.black]
 line-length = 120