Replace {distdir} with dist

[tom-tan]

This request is to fix #210.

As I mentioned in #210, the source of problem is that tox removed the distdir variable in 4.0.

An official document suggests to use $TOX_PACKAGE instead of {distdir}.
On the other hand, as reported in tox-dev/tox#2706, just replacing {distdir} with $TOX_PACKAGE does not work as we expected.

This request just replaces {distdir} with dist.
It is simple enough and I hope it works as we expected.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.50%. Comparing base (dd269f3) to head (e00edc1).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #212   +/-   ##
=======================================
  Coverage   80.50%   80.50%           
=======================================
  Files           8        8           
  Lines         790      790           
  Branches      201      201           
=======================================
  Hits          636      636           
- Misses        115      117    +2     
+ Partials       39       37    -2     

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[tom-tan]

Thanks!