Use version_file option of setuptools-scm to keep version info

Closes #1044

.gitignore

@@ -33,6 +33,9 @@ my/
 dist/
 build/
 
+# Generated by setuptools_scm
+mpmath/_version.py
+
 # Tox files
 .tox/
 

mpmath/__init__.py

@@ -1,7 +1,4 @@
-from importlib.metadata import version
-
-__version__ = version(__name__)
-del version
+from ._version import __version__
 
 import functools
 import sys

pyproject.toml

@@ -54,6 +54,7 @@ exclude = ['.eggs', '.git']
 max_line_length = 200
 
 [tool.setuptools_scm]
+version_file = "mpmath/_version.py"
 
 [tool.pytest.ini_options]
 testpaths = ['mpmath', 'docs']
@@ -66,7 +67,7 @@ timeout = 600
 
 [tool.coverage.run]
 branch = true
-omit = ['mpmath/tests/*']
+omit = ['mpmath/tests/*', 'mpmath/_version.py']
 patch = ["subprocess"]
 
 [tool.coverage.html]