Find python binary dynamically

[airtower-luna]

Some systems have a suitable Python interpreter, but not under the expected python binary name. E.g. on Debian(-ish) systems the Python 3 interpreter is called python3. This PR aims to support both.

Description

• The Makefile now searches for both python3 and python (using which), and uses the first one found.
• tests/scripts_retention.py uses sys.executable instead of hardcoded 'python'. That should work completely independent of the platform.

Motivation and Context

Fixes #670

Checklist

• I have read the CONTRIBUTING.md document and my PR follows change requirements therein
• the changes are also reflected in documentation and code comments
• all new and existing tests pass (see Travis CI results)
• test script checklist was followed for new scripts
• new test script added to tlslite-ng.json and tlslite-ng-random-subset.json
• new and modified scripts were ran against popular TLS implementations:
  • OpenSSL
  • NSS
  • GnuTLS
• required version of tlslite-ng updated in requirements.txt and README.md

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This change is 

[airtower-luna]

In addition to the problem described in #670 the "server_command" definitions as read from the JSON files also hardcode python as the interpreter. The current patch just replaces that with sys.executable in run_with_json(), would you prefer changing it to something that'd be more obviously a token, like the existing {command}? Would be just a search and replace run to use e.g. {python}. 🙂

[tomato42]

yes, probably search and replace on the json files should be enough

Reviewed 1 of 1 files at r1, 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @airtower-luna)

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tests/scripts_retention.py, line 200 at r2 (raw file):

        command = srv_conf["server_command"]
        if command[0] == 'python':
            command[0] = sys.executable

I think using {python} to indicate that it will be replaced is a cleaner solution—principle of least surprise and all that

[airtower-luna]

@tomato42: I've changed the server setup to use a {python} token, using a loop like the {command} token to avoid surprises in case there will ever be command components before the Python interpreter.

[tomato42]

Reviewed 3 of 3 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved

[tomato42]

looks good, thank you!