TST: Don't use 'is' on strings to avoid SyntaxWarning
This avoids the below warning (in Python 3.8, [user-visible in Debian](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=956021) because they byte-compile everything on install), and possibly unspecified behaviour (though I haven't seen that in practice).

```
  /usr/lib/python3/dist-packages/pandas/tests/frame/test_alter_axes.py:241: SyntaxWarning: "is" with a literal. Did you mean "=="?
    False if (keys[0] is "A" and keys[1] is "A") else drop  # noqa: F632
```

