refactor(setup.py): Introduce get_debian_version

Move getting the Debian package version into a separate function and
fail in case it cannot find it or fails parsing it.

Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>

setup.py

@@ -1,12 +1,23 @@
 #!/usr/bin/python3
 
 import glob
-import os
+import pathlib
 import re
 
 from setuptools import setup
 
 
+def get_debian_version() -> str:
+    """Look what Debian version we have."""
+    changelog = pathlib.Path(__file__).parent / "debian" / "changelog"
+    with changelog.open("r", encoding="utf-8") as changelog_f:
+        head = changelog_f.readline()
+    match = re.compile(r".*\((.*)\).*").match(head)
+    if not match:
+        raise ValueError(f"Failed to extract Debian version from '{head}'.")
+    return match.group(1)
+
+
 def make_pep440_compliant(version: str) -> str:
     """Convert the version into a PEP440 compliant version."""
     public_version_re = re.compile(r"^([0-9][0-9.]*(?:(?:a|b|rc|.post|.dev)[0-9]+)*)\+?")
@@ -19,14 +30,6 @@ def make_pep440_compliant(version: str) -> str:
     return pep440_version
 
 
-# look/set what version we have
-changelog = "debian/changelog"
-if os.path.exists(changelog):
-    head = open(changelog, "r", encoding="utf-8").readline()
-    match = re.compile(r".*\((.*)\).*").match(head)
-    if match:
-        version = match.group(1)
-
 scripts = [
     "backportpackage",
     "bitesize",
@@ -81,7 +84,7 @@ data_files = [
 if __name__ == "__main__":
     setup(
         name="ubuntu-dev-tools",
-        version=make_pep440_compliant(version),
+        version=make_pep440_compliant(get_debian_version()),
         scripts=scripts,
         packages=[
             "ubuntutools",