chore: add graceful exit

[fatelei]

What do these changes do?

graceful exit, even event loop is closed

Are there changes in behavior for the user?

nothing

Related issue number

fix #992

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.59%. Comparing base (1427b7b) to head (d3de89c).

Files with missing lines	Patch %	Lines
aiomysql/pool.py	0.00%	6 Missing ⚠️
aiomysql/sa/engine.py	33.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1041      +/-   ##
==========================================
- Coverage   93.74%   93.59%   -0.16%     
==========================================
  Files          36       36              
  Lines        5532     5542      +10     
  Branches      383      385       +2     
==========================================
+ Hits         5186     5187       +1     
- Misses        259      268       +9     
  Partials       87       87              

Flag	Coverage Δ
CI-GHA	93.19% <16.66%> (-0.16%)	⬇️
DB-mariadb-10.11	90.20% <16.66%> (-0.15%)	⬇️
DB-mariadb-10.6	90.20% <16.66%> (-0.15%)	⬇️
DB-mariadb-11.4	90.34% <16.66%> (-0.15%)	⬇️
DB-mariadb-11.8	90.34% <16.66%> (-0.15%)	⬇️
DB-mariadb-12.0	90.34% <16.66%> (-0.15%)	⬇️
DB-mysql-8.0	92.33% <16.66%> (-0.15%)	⬇️
DB-mysql-8.4	91.46% <16.66%> (-0.15%)	⬇️
DB-mysql-9.4	91.46% <16.66%> (-0.15%)	⬇️
OS-Linux	93.19% <16.66%> (-0.16%)	⬇️
Py-3.10.18	93.16% <16.66%> (-0.16%)	⬇️
Py-3.11.13	93.16% <16.66%> (-0.16%)	⬇️
Py-3.12.11	93.16% <16.66%> (-0.16%)	⬇️
Py-3.13.7	93.16% <16.66%> (-0.16%)	⬇️
Py-3.9.23	92.97% <16.66%> (-0.16%)	⬇️
VM-ubuntu-latest	93.19% <16.66%> (-0.16%)	⬇️
ubuntu-latest_3.10_mariadb-10.11	90.16% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.10_mariadb-10.6	90.16% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.10_mariadb-11.4	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.10_mariadb-11.8	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.10_mariadb-12.0	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.10_mysql-8.0	92.29% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.10_mysql-8.4	91.42% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.10_mysql-9.4	91.42% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.11_mariadb-10.11	90.16% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.11_mariadb-10.6	90.16% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.11_mariadb-11.4	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.11_mariadb-11.8	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.11_mariadb-12.0	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.11_mysql-8.0	92.29% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.11_mysql-8.4	91.42% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.11_mysql-9.4	91.42% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.12_mariadb-10.11	90.16% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.12_mariadb-10.6	90.16% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.12_mariadb-11.4	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.12_mariadb-11.8	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.12_mariadb-12.0	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.12_mysql-8.0	92.29% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.12_mysql-8.4	91.42% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.12_mysql-9.4	91.42% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.13_mariadb-10.11	90.16% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.13_mariadb-10.6	90.16% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.13_mariadb-11.4	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.13_mariadb-11.8	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.13_mariadb-12.0	90.31% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.13_mysql-8.0	92.29% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.13_mysql-8.4	91.42% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.13_mysql-9.4	91.42% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.9_mariadb-10.11	89.99% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.9_mariadb-10.6	89.99% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.9_mariadb-11.4	90.13% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.9_mariadb-11.8	90.13% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.9_mariadb-12.0	90.13% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.9_mysql-8.0	92.10% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.9_mysql-8.4	91.23% <16.66%> (-0.15%)	⬇️
ubuntu-latest_3.9_mysql-9.4	91.23% <16.66%> (-0.15%)	⬇️

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