remove global declaration for _instance_lock

[vnktshr21]

• This contribution adheres to CONTRIBUTING.md.
• I've updated CHANGELOG.md if applicable.
• I've added tests applicable for this pull request

What does this Pull Request accomplish?

• Removing the declaration of _instance_lock as global in the get() method since we are only reading it and not assigning value to it
  • Newer flake8 7.3.0 has started throwing F824 error for this declaration
  • See discussion here

List issues fixed by this Pull Request below, if any.

• Should fix the F824 flake error in Travis CI

What testing has been done?

Only tox build.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.18%. Comparing base (cf435c0) to head (eb318ba).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2144      +/-   ##
==========================================
- Coverage   89.62%   89.18%   -0.45%     
==========================================
  Files          61       71      +10     
  Lines       17308    18960    +1652     
==========================================
+ Hits        15513    16909    +1396     
- Misses       1795     2051     +256     

Flag	Coverage Δ
codegenunittests	84.44% <ø> (?)
nidcpowersystemtests	94.65% <ø> (+0.04%)	⬆️
nidcpowerunittests	89.53% <ø> (ø)
nidigitalsystemtests	92.26% <ø> (ø)
nidigitalunittests	68.44% <ø> (ø)
nidmmsystemtests	92.72% <ø> (ø)
nifakeunittests	85.52% <ø> (ø)
nifgensystemtests	94.61% <ø> (ø)
nimodinstsystemtests	73.85% <ø> (ø)
nimodinstunittests	94.20% <ø> (ø)
nirfsgsystemtests	76.53% <ø> (ø)
niscopesystemtests	92.94% <ø> (ø)
niscopeunittests	43.20% <ø> (ø)
nisesystemtests	91.50% <ø> (ø)
niswitchsystemtests	82.03% <ø> (ø)
nitclksystemtests	94.87% <ø> (ø)
nitclkunittests	98.26% <ø> (ø)

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see 11 files with indirect coverage changes

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