pyprog macro, added in 0ee52376 (Generalize how nxdtest python script is
generated into pyprog recipe macro) works by generating buildout code at
runtime in text form. This way it should be careful when substituting strings,
because if those strings contain \n, then intended control flow might become broken.

For example when using pyprog with eggs = ${eggs:eggs} from stack/erp5/ ,
buildout breaks because erp5 defines eggs as multiline list:

    INFO     self.buildout.parse("""
    INFO   File "/srv/slapgrid/slappart47/srv/runner/software/7f1663e8148f227ce3c6a38fc52796e2/eggs/zc.buildout-2.7.1+slapos020-py3.9.egg/zc/buildout/buildout.py", line 1352, in parse
    INFO     sections = zc.buildout.configparser.parse(
    INFO   File "/srv/slapgrid/slappart47/srv/runner/software/7f1663e8148f227ce3c6a38fc52796e2/eggs/zc.buildout-2.7.1+slapos020-py3.9.egg/zc/buildout/configparser.py", line 241, in parse
    INFO     raise e
    INFO zc.buildout.configparser.ParsingError: File contains parsing errors:
    INFO         [line 18]: 'python-barcode\n'
    INFO         [line 19]: 'SOAPpy-py3\n'
    INFO         [line 20]: 'suds-py3\n'
    INFO         [line 21]: 'neoppod[admin, ctl, master]\n'
    INFO         [line 22]: 'cython-zstd\n'
    INFO         [line 23]: 'msgpack\n'
    INFO         [line 24]: 'mysqlclient\n'
    INFO         [line 25]: 'PyMySQL\n'
    INFO         [line 26]: 'ZODB\n'
    INFO         [line 27]: 'zodbtools\n'
    INFO         [line 28]: 'psutil\n'
         ...

-> Fix it via indenting eggs list like we already do with pyinit code.

/cc @kazuhiko, @jerome