setup.py: source version dynamically to fix build from source

We use the first method from the documentation at
https://packaging.python.org/en/latest/guides/single-sourcing-package-version.

Thanks to Weiliang Li (Github: https://github.com/kigawas) for pointing
out the source build was broken and proposing a (different) fix.

Author: darosior

setup.py

@@ -1,6 +1,26 @@
-from setuptools import setup
-import bip32
 import io
+import os
+
+from setuptools import setup
+
+
+# Taken from https://github.com/pypa/pip/blob/003c7ac56b4da80235d4a147fbcef84b6fbc8248/setup.py#L7-L21
+def read(rel_path: str) -> str:
+    here = os.path.abspath(os.path.dirname(__file__))
+    # intentionally *not* adding an encoding option to open, See:
+    #   https://github.com/pypa/virtualenv/issues/201#issuecomment-3145690
+    with open(os.path.join(here, rel_path)) as fp:
+        return fp.read()
+
+
+# Taken from https://github.com/pypa/pip/blob/003c7ac56b4da80235d4a147fbcef84b6fbc8248/setup.py#L7-L21
+def get_version(rel_path: str) -> str:
+    for line in read(rel_path).splitlines():
+        if line.startswith("__version__"):
+            # __version__ = "0.9"
+            delim = '"' if '"' in line else "'"
+            return line.split(delim)[1]
+    raise RuntimeError("Unable to find version string.")
 
 
 with io.open("README.md", encoding="utf-8") as f:
@@ -10,7 +30,8 @@
     requirements = [r for r in f.read().split('\n') if len(r)]
 
 setup(name="bip32",
-      version=bip32.__version__,
+      # We use the first approach from https://packaging.python.org/en/latest/guides/single-sourcing-package-version
+      version=get_version("bip32/__init__.py"),
       description="Minimalistic implementation of the BIP32 key derivation scheme",
       long_description=long_description,
       long_description_content_type="text/markdown",