Remove span attributes pymemcache (#4076)

* opentelemetry-instrumentation-pymemcache: Remove usage of deprecated SpanAttributes

* Fixed linting issues

* Incorporated Review Comments for usage of NET_TRANSPORT

* Added back the empty lines

* Apply suggestions from code review

* Update __init__.py

* Update __init__.py

* Update test_pymemcache.py

* Executed tox

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>