fix(build): read version statically from brainpy._version

PR #865 moved __version__ into brainpy/_version.py and made __init__ re-export
it. That broke static version resolution for consumers still pointing at
brainpy.__init__:

- pyproject.toml used `attr = "brainpy.__version__"`. setuptools reads attr via
  AST only when it is a literal assignment; after the move __init__ exposes the
  name through an ImportFrom, so setuptools falls back to importing the whole
  brainpy package. In Publish.yml's isolated `python -m build` env (no jax /
  brainstate) that import fails. Point the attr at brainpy._version.__version__,
  which resolves statically without importing the package.
- docs/conf.py imported the full brainpy package solely to read
  brainpy.__version__. Read it directly from brainpy._version instead.

brainpy/__init__.py keeps re-exporting __version__/__version_info__, so
bp.__version__ still works for users.

Author: chaoming0625

docs/conf.py

@@ -56,9 +56,7 @@
 
 fix_ipython2_lexer_in_notebooks(os.path.dirname(os.path.abspath(__file__)))
 
-import brainpy
-
-release = brainpy.__version__
+from brainpy._version import __version__ as release
 
 # -- General configuration ---------------------------------------------------
 

pyproject.toml

@@ -82,7 +82,11 @@ exclude = [
 ]
 
 [tool.setuptools.dynamic]
-version = { attr = "brainpy.__version__" }
+# Read the version statically from the tiny ``brainpy/_version.py`` module. Pointing at
+# ``brainpy.__version__`` would force setuptools to import the whole package at build time
+# (``__init__`` only re-exports the name), which fails in the isolated ``python -m build``
+# environment where BrainPy's runtime deps are absent.
+version = { attr = "brainpy._version.__version__" }
 
 
 # ---------------------------------------------------------------------------