Use importlib.metadata to set __version__

This way we don't have to remember to update the version in two places
every release.  The version will only need to be set in pyproject.toml

backoff/__init__.py

@@ -12,7 +12,9 @@
 For examples and full documentation see the README at
 https://github.com/litl/backoff
 """
-from backoff._decorator import on_predicate, on_exception
+import importlib.metadata
+
+from backoff._decorator import on_exception, on_predicate
 from backoff._jitter import full_jitter, random_jitter
 from backoff._wait_gen import constant, expo, fibo, runtime
 
@@ -27,4 +29,4 @@
     'random_jitter',
 ]
 
-__version__ = '2.1.0'
+__version__ = importlib.metadata.version("backoff")