Fix leap year problem in age command

[u1-liquid]

생일이 윤년의 2월 29일인 경우 올해 생일을 계산하면서 ValueError 예외가 발생합니다.

Traceback

  File "/home/yui/yui/yui/bot.py", line 276, in handle
    return await handler.run(self, event)
  File "/home/yui/yui/yui/box/apps/basic.py", line 92, in run
    return await self._run_message_event(bot, event)
  File "/home/yui/yui/yui/box/apps/basic.py", line 180, in _run_message_event
    res = await self.handler(**kwargs)
  File "/home/yui/yui/yui/apps/date/age.py", line 50, in age
    this_year_birthday = birthday.replace(today.year)
ValueError: day is out of range for month

[codecov[bot]]

Codecov Report

Merging #136 into master will decrease coverage by 1.12%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
- Coverage   60.63%   59.51%   -1.13%     
==========================================
  Files         106      106              
  Lines        5538     5535       -3     
==========================================
- Hits         3358     3294      -64     
- Misses       2180     2241      +61

Impacted Files	Coverage Δ
yui/apps/date/age.py	100% <100%> (ø)	:arrow_up:
yui/apps/compute/translate.py	38% <0%> (-62%)	:arrow_down:
yui/apps/search/book.py	50% <0%> (-50%)	:arrow_down:
yui/apps/weather/aqi.py	82.95% <0%> (-17.05%)	:arrow_down:

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