migrate from setup.py to pyproject.toml (deepmodeling#995)

Follows PEP 621.
This PR also removes the automatic `__date__` -- it only reflects the
installation date.

dpgen/__init__.py

@@ -21,10 +21,6 @@
     from ._version import version as __version__
 except ImportError:
     __version__ = 'unkown'
-try:
-    from ._date import date as __date__
-except ImportError:
-    __date__ = 'unkown'
 
 def info():
     """

pyproject.toml

@@ -0,0 +1,49 @@
+[build-system]
+requires = ["setuptools>=45", "setuptools_scm[toml]>=6.2"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "dpgen"
+dynamic = ["version"]
+description = "DP-GEN: The deep potential generator"
+authors = [
+  {name = "DeepModeling"},
+  {name = "Han Wang", email = "[email protected]"},
+]
+license = {file = "LICENSE"}
+classifiers = [
+    "Programming Language :: Python :: 3.6",
+    "License :: OSI Approved :: GNU Lesser General Public License v3 (LGPLv3)",
+]
+dependencies = [
+    'numpy>=1.14.3',
+    'dpdata>=0.2.6',
+    'pymatgen>=2022.7.19',
+    'ase',
+    'monty>2.0.0',
+    'paramiko',
+    'custodian',
+    'GromacsWrapper>=0.8.0',
+    'dpdispatcher>=0.3.11',
+    'netCDF4',
+    'dargs>=0.2.9',
+    'h5py',
+    'pymatgen-analysis-defects',
+]
+requires-python = ">=3.6"
+readme = "README.md"
+keywords = ["deep potential generator", "active learning", "deepmd-kit"]
+
+[project.urls]
+Homepage = "https://github.com/deepmodeling/dpgen"
+documentation = "https://docs.deepmodeling.com/projects/dpgen"
+repository = "https://github.com/deepmodeling/dpgen"
+
+[project.entry-points.console_scripts]
+dpgen = "dpgen.main:main"
+
+[tool.setuptools.packages.find]
+include = ["dpgen*"]
+
+[tool.setuptools_scm]
+write_to = "dpgen/_version.py"