Qualify attribute names for generated ivar

[toslunar]

Subject: Make Napoleon qualify attribute names for generated ivar

Feature or Bugfix

• Feature

Purpose

This PR gives a partial fix of #4074. I hope this works in the most cases (an exception is the same attribute name in different classes with the same name, in python<3.3).

Detail

• Make Napoleon apply a workaround that my team has manually done.
  For example, the Attributes items in

class A(object):
    """
    An A.

    Attributes:
        foo (int): Blah blah.
    """

will be treated as

    Attributes:
        ~module_name.A.foo (int): Blah blah.

which is converted to

    :ivar ~module_name.A.foo: Blah blah.
    :vartype ~module_name.A.foo: int

in order to avoid unintended links.

• This PR does not break "manually fixed" lines.

Relates

• Prevent python class/instance variables from linking to objects #4676

[codecov]

Codecov Report

Merging #5129 into master will decrease coverage by 0.01%.
The diff coverage is 27.27%.

@@            Coverage Diff             @@
##           master    #5129      +/-   ##
==========================================
- Coverage   82.31%   82.29%   -0.02%     
==========================================
  Files         297      297              
  Lines       39212    39223      +11     
  Branches     6033     6035       +2     
==========================================
+ Hits        32276    32279       +3     
- Misses       5602     5610       +8     
  Partials     1334     1334

Impacted Files	Coverage Δ
sphinx/ext/napoleon/docstring.py	77.27% <27.27%> (-0.91%)	⬇️
sphinx/__init__.py	57.5% <0%> (-2.5%)	⬇️
sphinx/builders/html.py	82.73% <0%> (+0.1%)	⬆️

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[RobRuana]

This looks great! I would like to see at least one unit test that exercises the new code. Add a unit test, and I'll approve this!

[toslunar]

Thank you very much for refactoring and merging this!