util: Use tasklib.__version__ to determine module version

Importing pkg_resources module is notoriously slow, see [1,2]. Tasklib
module now provides __version__ attribute for an easy method of version
checking.

[1] pypa/setuptools#510
[2] pypa/setuptools#926

taskwiki/util.py

@@ -3,12 +3,13 @@
 from distutils.version import LooseVersion
 
 import contextlib
-import pkg_resources
 import os
 import random
 import sys
 import vim  # pylint: disable=F0401
 
+import tasklib
+
 from taskwiki.errors import TaskWikiException
 from taskwiki import regexp
 
@@ -371,8 +372,8 @@ def enforce_dependencies(cache):
     TASKWARRIOR_VERSION = '2.4.0'
 
     # Check tasklib version
-    tasklib_module_info = pkg_resources.get_distribution('tasklib')
-    tasklib_installed_version = LooseVersion(tasklib_module_info.version)
+    tasklib_module_version = getattr(tasklib, '__version__', '2.2.0')
+    tasklib_installed_version = LooseVersion(tasklib_module_version)
     tasklib_required_version = LooseVersion(TASKLIB_VERSION)
 
     if tasklib_required_version > tasklib_installed_version: