Avoid runtime setuptools dependency

Using `pkg_resources` meant that setuptools had to be available during
runtime, but this dependency was undeclared. Switch to
importlib.metadata which is in stdlib since Python 3.8.

Fixes: #8

setup.cfg

@@ -33,6 +33,7 @@ install_requires =
     click ~= 8.0
     click-config-file
     click-option-group
+    importlib-metadata; python_version<'3.8'
     platformdirs >= 2.1
     requests
     requests_oauthlib

src/strava_offline/__init__.py

@@ -1,7 +1,10 @@
-# isort: skip_file
-from pkg_resources import get_distribution, DistributionNotFound
 try:
-    __version__ = get_distribution(__name__).version
-except DistributionNotFound:
+    from importlib.metadata import version, PackageNotFoundError  # type: ignore # isort: skip
+except ImportError:
+    from importlib_metadata import version, PackageNotFoundError  # type: ignore # isort: skip
+
+try:
+    __version__ = version(__name__)
+except PackageNotFoundError:
     # package is not installed
     pass