Python: extract reference tags

[masatake]

No description provided.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (293f11e) 85.32% compared to head (3ffd74b) 85.35%.
Report is 49 commits behind head on master.

Files	Patch %	Lines
parsers/python.c	97.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3579      +/-   ##
==========================================
+ Coverage   85.32%   85.35%   +0.02%     
==========================================
  Files         231      231              
  Lines       55732    55853     +121     
==========================================
+ Hits        47555    47675     +120     
- Misses       8177     8178       +1     

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[masatake]

I must redefine the semantics of the scope field:

def foo():
    a.b()

In this case, for the reference tag b, how the scope filed should be: scope:foo or scope:a. I got the hint about this from @pragmaware ago. I must re-read his comment, and I must document it in "the reference tag design guide" or something.

As @AmaiKinono suggested, there are two types of reference tags: dependency reference tags and generic reference tags. The former one should be enabled by default. The latter one would be disabled by default. This also should be written in the guide.

[masatake]

Versioning and updating the man page for Python are needed for merging.