simplified version file format

Now we exec _kwant_version.py into a namespace and get
the version from that.

kwant/_common.py

@@ -68,14 +68,11 @@ def get_version_from_git():
 
 
 
-# version file contains comments (lines starting with '#')
-# followed by the version string on its own on a single line
+# populate the version_info dictionary with values stored in the version file
+version_info = {}
 with open(os.path.join(package_root, version_file), 'r') as f:
-    cruft="\"'\n\t\r "  # strip whitespace and quotes
-    line = next(f).strip(cruft)
-    while line.startswith('#'):
-        line = next(f).strip(cruft)
-    version = line
+    exec(f.read(), {}, version_info)
+version = version_info['version']
 version_is_from_git = (version == "__use_git__")
 if version_is_from_git:
     version = get_version_from_git()

kwant/_kwant_version.py

 # This file will be overwritten by setup.py when a source or binary
 # distribution is made.  The magic value "__use_git__" is interpreted by
 # _common.py in this directory.
-"__use_git__"
+version = "__use_git__"

setup.py

@@ -173,7 +173,7 @@ def git_lsfiles():
 
     if p.wait() != 0:
         return
-    return p.communicate()[0].split('\n')[:-1]
+    return p.communicate()[0].decode().split('\n')[:-1]
 
 
 # Make the command "sdist" depend on "build".  This verifies that the
@@ -235,7 +235,7 @@ def write_version(fname):
         pass
     with open(fname, 'w') as f:
         f.write("# This file has been created by setup.py.\n")
-        f.write(version)
+        f.write("version = '{}'".format(version))
 
 
 def long_description():