Fix bug: use sys instead of inspect to obtain stack

`sys` has less dependencies than `inspect` and is trouble-less

lambdex/__init__.py

@@ -1,18 +1,17 @@
 import os
 import sys
-import inspect
 
-for f in inspect.stack()[1:]:
-    if not f.filename.startswith('<'):
-        break
+f = sys._getframe(1)
+while f.f_code is not None and f.f_code.co_filename.startswith('<'):
+    f = f.f_back
 
-if f.filename.startswith(os.path.dirname(os.__file__)):
+if f and f.f_code and f.f_code.co_filename.startswith(os.path.dirname(os.__file__)):
     if sys.path and sys.path[0] == os.getcwd():
         sys.path = sys.path[1:]
 else:
     __all__ = ['def_']
     from .keywords import def_
 
-del os, sys, inspect, f
+del os, sys, f
 
 __version__ = "0.1.0"