update version.py

should fix #23

.gitattributes

@@ -0,0 +1 @@
+version.py  export-subst

version.py

@@ -1,27 +1,37 @@
-# This program is placed into the public domain.
-
-__all__ = ('get_version')
+# Source: https://github.com/Changaco/version.py
 
 from os.path import dirname, isdir, join
 import re
 from subprocess import CalledProcessError, check_output
 
+__all__ = ('get_version')
+
+tag_re = re.compile(r'\btag: ([0-9][^,]*)\b')
 version_re = re.compile('^Version: (.+)$', re.M)
 
+
 def get_version():
+    # Return the version if it has been injected into the file by git-archive
+    version = tag_re.search('$Format:%D$')
+    if version:
+        return version.group(1)
+
     d = dirname(__file__)
 
     if isdir(join(d, '.git')):
         # Get the version using "git describe".
-        cmd = 'git describe --tags --match [0-9]*'.split()
+        cmd = 'git describe --tags --match [0-9]* --dirty'.split()
         try:
             version = check_output(cmd).decode().strip()
         except CalledProcessError:
             print('Unable to get version number from git tags')
             exit(1)
 
-        # PEP 386 compatibility
+        # PEP 440 compatibility
         if '-' in version:
+            if version.endswith('-dirty'):
+                print('The working tree is dirty')
+                exit(1)
             version = '.post'.join(version.split('-')[:2])
 
     else: