Python tests: avoid using "is" to compare literals
Python tests: avoid using "is" to compare literals

Python 3.8 emits a SyntaxWarning when "is" is used to compare literals.

  test/py/ganeti.serializer_unittest.py:205: SyntaxWarning: "is" with a literal. Did you mean "=="?
    self.assertTrue("egg" is pDict["bar"].Get()

"is" checks the object identities and may work with some literals that
are singletons (e.g. strings), but may not work with others. Since it's
string equality that we care about, we should use "==" rather than "is".

Signed-off-by: Apollon Oikonomopoulos <ed81ed0a45ccd5c8869bda52e38353a6299ee39e@dmesg.gr>

ganeti/ganeti,mbakke/ganeti,mbakke/ganeti,ganeti/ganeti,mbakke/ganeti,ganeti/ganeti