connection: add a pointer to os.getpid to ConnectionPool

[mkmkme]

This commit adds a local pointer to os.getpid to ConnectionPool class to mitigate the AttributeError that occurs on Python 3.13+ on interpreter exit. This is caused by the fact that interpreter calls __del__ on variables on exit, but os is being destroyed before the valkey.Valkey instance.

It could be easily reproduced with:

>>> import valkey
>>> r = valkey.Valkey(host="localhost", port=6379)
>>> exit()
Exception ignored in: <function Valkey.__del__ at 0x7f29d084e5c0>
Traceback (most recent call last):
  File "/home/mikhail.koviazin/workspace/valkey-py-clean/valkey/client.py", line 521, in __del__
  File "/home/mikhail.koviazin/workspace/valkey-py-clean/valkey/client.py", line 536, in close
  File "/home/mikhail.koviazin/workspace/valkey-py-clean/valkey/connection.py", line 1179, in disconnect
  File "/home/mikhail.koviazin/workspace/valkey-py-clean/valkey/connection.py", line 1083, in _checkpid
TypeError: 'NoneType' object is not callable

Having a local pointer to that function keeps os.getpid() accessible during interpreter shutdown.

Fixes #158

Pull Request check-list

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

Description of change

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.23%. Comparing base (ebdf9c9) to head (4bdeb90).

Files with missing lines	Patch %	Lines
valkey/connection.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #159   +/-   ##
=======================================
  Coverage   76.22%   76.23%           
=======================================
  Files         130      130           
  Lines       33946    33947    +1     
=======================================
+ Hits        25877    25878    +1     
  Misses       8069     8069           

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