Disable smoke tests against Python 2.7

[DonJayamanne]

Smoke tests only existed on Python 3.7 and Azure seems to be very flaky when running tests again Python 2.7 on Linux (Azure hangs pretty often, even when tests have completed).

[codecov-io]

Codecov Report

Merging #7774 into master will decrease coverage by 0.69%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #7774     +/-   ##
=========================================
- Coverage   58.75%   58.06%   -0.7%     
=========================================
  Files         496      496             
  Lines       22157    21796    -361     
  Branches     3570     3565      -5     
=========================================
- Hits        13018    12655    -363     
- Misses       8321     8323      +2     
  Partials      818      818

Impacted Files	Coverage Δ
...nt/datascience/jupyter/invalidNotebookFileError.ts	0% <0%> (-75%)	⬇️
.../client/datascience/jupyter/jupyterConnectError.ts	0% <0%> (-66.67%)	⬇️
...science/jupyter/jupyterKernelPromiseFailedError.ts	0% <0%> (-50%)	⬇️
...ent/datascience/jupyter/jupyterWaitForIdleError.ts	0% <0%> (-50%)	⬇️
src/client/language/characters.ts	12% <0%> (-21.34%)	⬇️
src/client/testing/codeLenses/main.ts	60% <0%> (-17.78%)	⬇️
src/client/datascience/cellFactory.ts	72.34% <0%> (-17.32%)	⬇️
src/client/constants.ts	75% <0%> (-5%)	⬇️
src/client/providers/providerUtilities.ts	84.61% <0%> (-4.28%)	⬇️
src/client/api.ts	90% <0%> (-2.86%)	⬇️
... and 25 more

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