build: extract version number in setup.py from version.py

setup.py

@@ -1,10 +1,16 @@
 from setuptools import setup, find_packages
 from pathlib import Path
+from distutils.util import convert_path
+
 this_directory = Path(__file__).parent
 long_description = (this_directory / "README.md").read_text()
 
+version = {}
+with open(convert_path('pyerrors/version.py')) as ver_file:
+    exec(ver_file.read(), version)
+
 setup(name='pyerrors',
-      version='2.2.0+dev',
+      version=version['__version__'],
       description='Error analysis for lattice QCD',
       long_description=long_description,
       long_description_content_type='text/markdown',