Fix example tests on Win32.

[Oberon00]

They were previously failing with

OSError: [WinError 193] %1 is not a valid Win32 application

Since subprocess.Popen directly calls CreateProcess and the Windows
kernel doesn't support shebangs (even if it did, /usr/bin/env is not a
path that leads to a valid executable usually), we need to manually
specify sys.executable ('python' would not work since it would leave the
virtualenv).

[codecov-io]

Codecov Report

Merging #302 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #302   +/-   ##
=======================================
  Coverage   85.65%   85.65%           
=======================================
  Files          33       33           
  Lines        1610     1610           
  Branches      180      180           
=======================================
  Hits         1379     1379           
  Misses        185      185           
  Partials       46       46

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[codeboten]

Thanks for catching this!