chore: Use git tag for package version

Update `setup.py` to use git tag for package version.

setup.py

@@ -1,23 +1,39 @@
 #!/usr/bin/python
 import os
 import re
+import subprocess
 from setuptools import setup
 
 with open("README.md", encoding="utf-8") as f:
     long_description = f.read()
 
-# Attempt to get version number from TravisCI environment variable
-version = os.environ.get("TRAVIS_TAG", default="0.0.0")
 
-# Remove leading 'v'
-version = re.sub("^v", "", version)
+def get_version():
+    """Get the version from git tags.
+
+    Version is determined by the latest git tag, and will be the tag name without the leading 'v'.
+
+    Returns:
+        str: The version number.
+    """
+    try:
+        # Get the latest git tag
+        version = subprocess.check_output(
+            ["git", "describe", "--tags", "--abbrev=0"], encoding="utf-8"
+        ).strip()
+        version = re.sub("^v", "", version)
+        return version
+
+    except subprocess.CalledProcessError:
+        return "0.0.0"
+
 
 setup(
     name="anybadge",
     description="Simple, flexible badge generator for project badges.",
     long_description=long_description,
     long_description_content_type="text/markdown",
-    version=version,
+    version=get_version(),
     author="Jon Grace-Cox",
     author_email="30441316+jongracecox@users.noreply.github.com",
     packages=["anybadge", "anybadge.templates", "anybadge.server"],