conf.py: set the version for sphinx, based on the current git tag

conf.py

@@ -20,6 +20,7 @@
 import json
 import logging
 import os
+import re
 import subprocess
 import sys
 import urllib.parse
@@ -106,7 +107,25 @@ copyright = '2014-2020, MicroPython & CircuitPython contributors (https://github
 #
 # We don't follow "The short X.Y version" vs "The full version, including alpha/beta/rc tags"
 # breakdown, so use the same version identifier for both to avoid confusion.
-version = release = '0.0.0'
+
+final_version = ""
+git_describe = subprocess.run(
+    ["git", "describe", "--dirty", "--tags"],
+    stdout=subprocess.PIPE,
+    stderr=subprocess.STDOUT,
+    encoding="utf-8"
+)
+if git_describe.returncode == 0:
+    git_version = re.search(
+        r"^\d(?:\.\d){0,2}(?:\-(?:alpha|beta)\.\d+){0,1}",
+        str(git_describe.stdout)
+    )
+    if git_version:
+        final_version = git_version[0]
+else:
+    print("Failed to retrieve git version:", git_describe.stdout)
+
+version = release = final_version
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.