makeversionhdr should be unforgiving

Recently(?) github started making it the default to only copy a single
branch (e.g., main) and NO TAGS into new forks.

This makes the step of the build process that determines the CircuitPython
version not work, because tags are expected to be present. When tags are
not present, the version number is only a git hash. The version number
ends up being 0.0.0.

This causes problems with libraries that check for CircuitPython version
to determine compatibility, among other things.

We'll do other things to improve the situation, such as document it.
But it'd also be good if the build stopped when this detectable condition
occurs.

py/makeversionhdr.py

@@ -59,29 +59,25 @@ def get_version_info_from_git():
     return git_tag, git_hash, ver
 
 
-def get_version_info_from_docs_conf():
-    with open(os.path.join(os.path.dirname(sys.argv[0]), "..", "conf.py")) as f:
-        for line in f:
-            if line.startswith("version = release = '"):
-                ver = line.strip().split(" = ")[2].strip("'")
-                git_tag = "v" + ver
-                ver = ver.split(".")
-                if len(ver) == 2:
-                    ver.append("0")
-                return git_tag, "<no hash>", ver
-    return None
+def cannot_determine_version():
+    raise SystemExit(
+        """Cannot determine version.
+
+CircuitPython must be built from a git clone with tags.
+If you cloned from a fork, fetch the tags from adafruit/circuitpython as follows:
+
+    git fetch --tags --recurse-submodules=no --shallow-since="2021-07-01" https://github.com/adafruit/circuitpython HEAD"""
+    )
 
 
 def make_version_header(filename):
-    # Get version info using git, with fallback to docs/conf.py
+    # Get version info using git (required)
     info = get_version_info_from_git()
     if info is None:
-        info = get_version_info_from_docs_conf()
-
+        cannot_determine_version()
     git_tag, git_hash, ver = info
     if len(ver) < 3:
-        ver = ("0", "0", "0")
-        version_string = git_hash
+        cannot_determine_version()
     else:
         version_string = ".".join(ver)