.travis.yml: pin setuptools to 49.6 on pypy3

[perrinjerome]

setutools 50.0.0 does not seem compatible with the pypy3 used on travis.

[perrinjerome]

https://travis-ci.org/github/zopefoundation/ZODB/jobs/721278991#L217 was a run from last week which succeeded for pyp3, setuptools 49.6.0 was selected. This week the same job https://travis-ci.org/github/zopefoundation/ZODB/jobs/722605925#L214 picks setuptools 50.0.0 with which buildout errors:

$ buildout $BUILOUT_OPTIONS
Traceback (most recent call last):
  File "/home/travis/virtualenv/pypy3.6-7.1.1/bin/buildout", line 5, in <module>
    from zc.buildout.buildout import main
  File "/home/travis/virtualenv/pypy3.6-7.1.1/site-packages/zc/buildout/buildout.py", line 18, in <module>
    import zc.buildout.easy_install
  File "/home/travis/virtualenv/pypy3.6-7.1.1/site-packages/zc/buildout/easy_install.py", line 21, in <module>
    import distutils.errors
  File "/home/travis/virtualenv/pypy3.6-7.1.1/site-packages/_distutils_hack/__init__.py", line 82, in create_module
    return importlib.import_module('._distutils', 'setuptools')
  File "/home/travis/virtualenv/pypy3.6-7.1.1/lib-python/3/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 974, in _gcd_import
  File "<frozen importlib._bootstrap>", line 924, in _sanity_check
SystemError: Parent module 'setuptools' not loaded, cannot perform relative import

I quickly tried pypy v7.3.1 from https://www.pypy.org/download.html and it does not seem to have this problem, so probably once travis ci updates pypy this version pin will no longer be necessary.

[icemac]

LGTM