parse the version file instead of importing

Python 3 no longer allows implicit relative imports:
https://docs.python.org/release/3.0.1/whatsnew/3.0.html#removed-syntax
Instead of importing the version file we just parse it. By keeping
the extension ".py" we make it so that it will be distributed along
with the rest of the package.

kwant/_common.py

@@ -11,7 +11,9 @@ import os
 
 __all__ = ['version', 'KwantDeprecationWarning']
 
-distr_root = os.path.dirname(os.path.dirname(os.path.realpath(__file__)))
+package_root = os.path.dirname(os.path.realpath(__file__))
+distr_root = os.path.dirname(package_root)
+version_file = '_kwant_version.py'
 
 def get_version_from_git():
     try:
@@ -66,7 +68,14 @@ def get_version_from_git():
 
 
 
-from _kwant_version import version
+# version file contains comments (lines starting with '#')
+# followed by the version string on its own on a single line
+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
 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.
-version = "__use_git__"
+"__use_git__"

setup.py

@@ -47,11 +47,6 @@ TUT_HIDDEN_PREFIX = '#HIDDEN'
 # it is not built yet.
 _dont_write_bytecode_saved = sys.dont_write_bytecode
 sys.dont_write_bytecode = True
-try:
-    imp.load_source(STATIC_VERSION_PATH[-1].split('.')[0],
-                    os.path.join(*STATIC_VERSION_PATH))
-except IOError:
-    pass
 _common = imp.load_source('_common', 'kwant/_common.py')
 sys.dont_write_bytecode = _dont_write_bytecode_saved
 
@@ -228,7 +223,7 @@ 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 = '{}'\n".format(version))
+        f.write(version)
 
 
 def long_description():