Upload coverage only for python 3.8 on `ubuntu-latest`

[czgdp1807]

References to other Issues or PRs or Relevant literature

Closes #453

Brief description of what is fixed or changed

Other comments

[codecov[bot]]

Codecov Report

Merging #459 (6eeedf7) into master (87e0d56) will decrease coverage by 0.053%. The diff coverage is n/a.

@@              Coverage Diff              @@
##            master      #459       +/-   ##
=============================================
- Coverage   98.603%   98.549%   -0.054%     
=============================================
  Files           29        29               
  Lines         3795      3792        -3     
=============================================
- Hits          3742      3737        -5     
- Misses          53        55        +2     

Impacted Files	Coverage Δ
pydatastructs/trees/heaps.py	97.536% <0.000%> (-0.512%)	:arrow_down:
...datastructs/linear_data_structures/linked_lists.py	98.930% <0.000%> (-0.268%)	:arrow_down:
pydatastructs/graphs/algorithms.py	99.501% <0.000%> (-0.002%)	:arrow_down: