Make the plugin to be compatible with Python 3.

Squashed commit of the following:

commit 0973c5179e504ffbd74a38d6557bb49fe3bf8b5d
Author: Hong Xu <hong@topbug.net>
Date:   Sun Jan 10 00:11:11 2016 -0800

    Some minor corrections

commit ca17e97
Author: Shunsuke Shimizu <grafi@grafi.jp>
Date:   Sun Jan 10 15:24:13 2016 +0900

    vim 7.3 support by using `py[3]` command instead of `py[3]eval()` function

commit c51ae80
Author: grafi <grafi@grafi.jp>
Date:   Sun Jan 10 14:42:38 2016 +0900

    use print_function on python2

commit 401a948
Author: grafi <grafi@grafi.jp>
Date:   Sun Jan 10 14:33:25 2016 +0900

    assure that sys.path is cleaned

commit f3bf442
Author: grafi <grafi@grafi.jp>
Date:   Sun Jan 10 14:22:45 2016 +0900

    python3 style print

commit 8e05937
Author: Shunsuke Shimizu <grafi@grafi.jp>
Date:   Sat Dec 26 07:54:49 2015 +0900

    Support python3

plugin/editorconfig.py

@@ -0,0 +1,42 @@
+from __future__ import print_function
+
+try:
+    try:
+        import vim
+        import sys
+    except:
+        vim.command('let l:ret = 2')
+        raise
+
+    try:
+        sys.path.insert(0, vim.eval('a:editorconfig_core_py_dir'))
+
+        import editorconfig
+        import editorconfig.exceptions as editorconfig_except
+    except:
+        vim.command('let l:ret = 3')
+        raise
+    finally:
+        del sys.path[0]
+
+    # `ec_` prefix is used in order to keep clean Python namespace
+    ec_data = {}
+
+    def ec_UseConfigFiles():
+        ec_data['filename'] = vim.eval("expand('%:p')")
+        ec_data['conf_file'] = ".editorconfig"
+
+        try:
+            ec_data['options'] = editorconfig.get_properties(ec_data['filename'])
+        except editorconfig_except.EditorConfigError as e:
+            if int(vim.eval('g:EditorConfig_verbose')) != 0:
+                print(str(e), file=sys.stderr)
+            vim.command('let l:ret = 1')
+            return
+
+        for key, value in ec_data['options'].items():
+            vim.command("let l:config['" + key.replace("'", "''") + "'] = " +
+                "'" + value.replace("'", "''") + "'")
+
+except:
+    pass

plugin/editorconfig.vim

@@ -36,6 +36,8 @@ let g:loaded_EditorConfig = 1
 let s:saved_cpo = &cpo
 set cpo&vim
 
+let s:pyscript_path = expand('<sfile>:p:r') . '.py'
+
 " variables {{{1
 if !exists('g:EditorConfig_exec_path')
     let g:EditorConfig_exec_path = ''
@@ -231,48 +233,23 @@ function! s:InitializePythonBuiltin(editorconfig_core_py_dir) " {{{2
 
     let s:builtin_python_initialized = 1
 
-    let l:ret = 0
-
-    if !has('python')
+    if has('python')
+        let s:pyfile_cmd = 'pyfile'
+        let s:py_cmd = 'py'
+    elseif has('python3')
+        let s:pyfile_cmd = 'py3file'
+        let s:py_cmd = 'py3'
+    else
         return 1
     endif
 
-    python << EEOOFF
-
-try:
-    import vim
-    import sys
-except:
-    vim.command('let l:ret = 2')
-
-EEOOFF
-
-    if l:ret != 0
-        return l:ret
-    endif
-
-    python << EEOOFF
-
-try:
-    sys.path.insert(0, vim.eval('a:editorconfig_core_py_dir'))
-
-    import editorconfig
-    import editorconfig.exceptions as editorconfig_except
-
-except:
-    vim.command('let l:ret = 3')
-
-del sys.path[0]
-
-ec_data = {}  # used in order to keep clean Python namespace
-
-EEOOFF
-
-    if l:ret != 0
-        return l:ret
-    endif
+    let l:ret = 0
+    " The following line modifies l:ret. This is a bit confusing but
+    " unfortunately to be compatible with Vim 7.3, we cannot use pyeval. This
+    " should be changed in the future.
+    execute s:pyfile_cmd s:pyscript_path
 
-    return 0
+    return l:ret
 endfunction
 
 " Do some initalization for the case that the user has specified core mode {{{1
@@ -388,41 +365,22 @@ augroup END
 function! s:UseConfigFiles_Python_Builtin() " {{{2
 " Use built-in python to run the python EditorConfig core
 
-    let l:config = {}
-    let l:ret = 0
-
     " ignore buffers that do not have a file path associated
     if empty(expand('%:p'))
         return 0
     endif
 
-    python << EEOOFF
-
-ec_data['filename'] = vim.eval("expand('%:p')")
-ec_data['conf_file'] = ".editorconfig"
-
-try:
-    ec_data['options'] = editorconfig.get_properties(ec_data['filename'])
-except editorconfig_except.EditorConfigError as e:
-    if int(vim.eval('g:EditorConfig_verbose')) != 0:
-        print >> sys.stderr, str(e)
-    vim.command('let l:ret = 1')
+    let l:config = {}
 
-EEOOFF
+    let l:ret = 0
+    execute s:py_cmd 'ec_UseConfigFiles()'
     if l:ret != 0
         return l:ret
     endif
 
-    python << EEOOFF
-for key, value in ec_data['options'].items():
-    vim.command("let l:config['" + key.replace("'", "''") + "'] = " +
-        "'" + value.replace("'", "''") + "'")
-
-EEOOFF
-
     call s:ApplyConfig(l:config)
 
-    return 0
+    return l:ret
 endfunction
 
 function! s:UseConfigFiles_Python_External() " {{{2