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

Diff for: RELEASE.rst

@@ -19,7 +19,7 @@ After that is done, you may release the project by following these steps:
 
    #. Create the release commit
 
-      #. Bump the versions in ``meson.build`` and ``mesonpy/__init__.py``.
+      #. Bump the versions in ``pyproject.toml``.
       #. Create ``CHANGELOG.rst`` section for the new release and fill it.
       #. The commit message should read: ``REL: set version to X.Y.Z``
 

Diff for: 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()
 

Diff for: 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
 

Diff for: 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',