[issue4016] improve linecache: reuse tokenize.detect_encoding() and io.open()

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

@benjamin.peterson: The second version of my patch works correctly 
with the bootstraping.

> I also think we should consider hard adding more modules 
> that are loaded at startup time to py3k already huge list.

linecache is not loaded at startup time in py3k! I see that linecache 
is loaded by the warnings module, but the warnings module 
(Lib/warnings.py) is not loaded at startup. It was maybe the case with 
Python 2.x or older version of Python 3.x?

With my patch, loading linecache loads 2 extra modules: tokenize and 
token. It only impacts code using directly linecache or the warnings 
module.

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