add Pool.closed property as present in aiopg, fixes #463

[Nothing4You]

What do these changes do?

New closed property for Pool instances, exposing whether the pool is closed.

Are there changes in behavior for the user?

New property available.

Related issue number

Fixes #463

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment to CHANGES.txt

[codecov]

Codecov Report

Merging #726 (e6c0aa9) into master (00d0932) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #726   +/-   ##
=======================================
  Coverage   92.71%   92.71%           
=======================================
  Files          35       35           
  Lines        5448     5452    +4     
  Branches      918      919    +1     
=======================================
+ Hits         5051     5055    +4     
  Misses        309      309           
  Partials       88       88           

Flag	Coverage Δ
ubuntu-latest_3.10_mariadb-10.2	89.49% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.10_mariadb-10.3	89.49% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.10_mariadb-10.4	89.49% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.10_mariadb-10.5	89.47% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.10_mariadb-10.6	89.47% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.10_mariadb-10.7	89.47% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.10_mysql-5.7	90.18% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.10_mysql-8.0	91.59% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.11-dev_mariadb-10.2	89.45% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.11-dev_mariadb-10.3	89.45% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.11-dev_mariadb-10.4	89.45% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.11-dev_mariadb-10.5	89.43% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.11-dev_mariadb-10.6	89.43% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.11-dev_mariadb-10.7	89.43% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.11-dev_mysql-5.7	90.15% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.11-dev_mysql-8.0	91.56% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.7_mariadb-10.2	88.89% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.7_mariadb-10.3	88.89% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.7_mariadb-10.4	88.89% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.7_mariadb-10.5	88.87% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.7_mariadb-10.6	88.87% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.7_mariadb-10.7	88.87% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.7_mysql-5.7	89.63% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.7_mysql-8.0	91.13% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.8_mariadb-10.2	89.53% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.8_mariadb-10.3	89.53% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.8_mariadb-10.4	89.53% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.8_mariadb-10.5	89.51% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.8_mariadb-10.6	89.51% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.8_mariadb-10.7	89.51% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.8_mysql-5.7	90.22% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.8_mysql-8.0	91.62% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.9_mariadb-10.2	89.40% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.9_mariadb-10.3	89.40% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.9_mariadb-10.4	89.40% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.9_mariadb-10.5	89.38% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.9_mariadb-10.6	89.38% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.9_mariadb-10.7	89.38% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.9_mysql-5.7	90.10% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.9_mysql-8.0	91.49% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiomysql/pool.py	97.04% <100.00%> (+0.05%)	⬆️
tests/test_pool.py	97.38% <100.00%> (+<0.01%)	⬆️

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