Check if python starts python3

[pavoljuhas]

Problem: dev_tools/pypath does not work when python command does
not exist causing a failure in check/pytest (PYTHONPATH not set).

Solution: Require that python starts Python 3.
Bail out from the check scripts otherwise.

[pavoljuhas]

cc: @nafaynajam

[codecov]

Codecov Report

Patch coverage: 75.00% and no project coverage change.

Comparison is base (30fa6f5) 97.38% compared to head (72b5b2e) 97.38%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6239   +/-   ##
=======================================
  Coverage   97.38%   97.38%           
=======================================
  Files        1116     1116           
  Lines       96110    96118    +8     
=======================================
+ Hits        93594    93603    +9     
+ Misses       2516     2515    -1     

Files Changed	Coverage Δ
dev_tools/notebooks/isolated_notebook_test.py	64.61% <50.00%> (-0.47%)	⬇️
dev_tools/packaging/isolated_packages_test.py	60.00% <50.00%> (-1.12%)	⬇️
cirq-core/cirq/vis/heatmap_test.py	97.53% <100.00%> (-0.48%)	⬇️
dev_tools/cloned_env_test.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[nafaynajam]

@pavoljuhas I see that you have mentioned me in this issue, could you elaborate how I may be of help?, I see commits in code coverage, is it just to notify me of the changes in it?.

[pavoljuhas]

@pavoljuhas I see that you have mentioned me in this issue, could you elaborate how I may be of help?

Hi Nafay, this PR is to address the error on running check/pytest you asked about last Friday.
It makes the script terminate early with an informative error message rather than fail later on.

No action is needed, the cc is to let you know about the fix.
That said feel free to check the PR and post comments/suggestions if any.

[nafaynajam]

@pavoljuhas Very nice, thanks :-)