MAINT: keep version number only in pyproject.toml

mesonpy is not supposed to be used as a library, thus having the
version number in the __version__ attribute is not that useful.
Keeping the version number in only one place makes it impossible to
fail to keep the version number in sync. Having the version number in
pyproject.toml along the other metadata is the most obvious thing.

Author: dnicolodi

meson.build

@@ -2,7 +2,7 @@
 #
 # SPDX-License-Identifier: MIT
 
-project('meson-python', version: '0.18.0.dev0')
+project('meson-python')
 
 py = import('python').find_installation()
 

mesonpy/__init__.py

@@ -65,9 +65,6 @@
     MesonArgs = Mapping[MesonArgsKeys, List[str]]
 
 
-__version__ = '0.18.0.dev0'
-
-
 _NINJA_REQUIRED_VERSION = '1.8.2'
 _MESON_REQUIRED_VERSION = '0.63.3' # keep in sync with the version requirement in pyproject.toml
 

pyproject.toml

@@ -15,6 +15,7 @@ requires = [
 
 [project]
 name = 'meson-python'
+version = '0.18.0.dev0'
 description = 'Meson Python build backend (PEP 517)'
 readme = 'README.rst'
 requires-python = '>= 3.7'
@@ -40,10 +41,6 @@ dependencies = [
   'tomli >= 1.0.0; python_version < "3.11"',
 ]
 
-dynamic = [
-  'version',
-]
-
 [project.optional-dependencies]
 test = [
   'build',