Fix another SyntaxWarning where 'is' is used rather than '=='

[PureTryOut]

Seems another one slipped in somewhere in the last release.

Without this MR at least Python 3.8 will complain:

/usr/lib/python3.8/site-packages/lingua_franca/lang/format_cs.py:296: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if abs(num) < 1.0 and (result is "mínus " or not result):

Continuation of #112 I guess.

[ChanceNCounter]

Saw that last night! I was afraid I'd reintroduced it in a feature branch, so there's a sick sort of relief noticing that it's in master.

Nicely done, as always. Will double-check and merge.