gh-112192: Increase the trace module coverage precision to one decimal

[rruuaanng]

Before with the current trace module, you'll have a summary output like this:

lines   cov%   module   (path)
  154    24%   _weakrefset   (/home/gaogaotiantian/programs/mycpython/Lib/_weakrefset.py)
 1293     0%   ast   (/home/gaogaotiantian/programs/mycpython/Lib/ast.py)
  160     5%   bz2   (/home/gaogaotiantian/programs/mycpython/Lib/bz2.py)
  133    39%   cProfile   (/home/gaogaotiantian/programs/mycpython/Lib/cProfile.py)
  903     7%   collections.__init__   (/home/gaogaotiantian/programs/mycpython/Lib/collections/__init__.py)

This patch increase the precision of cov% to one decimal place so you'll have something like:

lines    cov%   module   (path)
  154   24.7%   _weakrefset   (/home/gaogaotiantian/programs/mycpython/Lib/_weakrefset.py)
 1293    0.2%   ast   (/home/gaogaotiantian/programs/mycpython/Lib/ast.py)
  160    5.0%   bz2   (/home/gaogaotiantian/programs/mycpython/Lib/bz2.py)
  133   39.8%   cProfile   (/home/gaogaotiantian/programs/mycpython/Lib/cProfile.py)
  903    7.8%   collections.__init__   (/home/gaogaotiantian/programs/mycpython/Lib/collections/__init__.py)

So, it makes the impact of small changes visible ;)

This may seem a bit non-sense at the first glance, but it's related to a very practical question - how do we know our coverage improved after some new test cases? With integer percentage, we have a 1% resolution, which means we can't tell if there's a one line improvement if the file is larger than 100 lines. For a file that has 1000 lines, the resolution is 10 lines! It's not trivial!

• Issue: Increase the precision of the summary in trace module #112192

[skirpichev]

Mostly LGTM, with one comment.

Also, you could change pr description to be more suitable as commit message. Just mention what you do and why it's useful.

[rruuaanng]

cc @erlend-aasland
For this issue and PR. WDYT? In the discussion, it was specifically mentioned that they hoped to get advice from core developers :)

[erlend-aasland]

Thanks for the PR, RUANG; thanks for the review Sergey.