build: add pyememcache dependency

[UsamaSadiq]

Description

• Under the issue Replace MemcachedCache backend with PymemcacheCache in analytics-api #665, adding the pymemcache to the dependencies.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (ceaec78) 97.52% compared to head (20d7e54) 97.52%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #673   +/-   ##
=======================================
  Coverage   97.52%   97.52%           
=======================================
  Files          54       54           
  Lines        3026     3026           
  Branches      266      266           
=======================================
  Hits         2951     2951           
  Misses         63       63           
  Partials       12       12           

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