[sigzero]

It is not T'is, it is Tis. No apostrophe.

[IncRnd]

The word is 'tis with an apostrophe.

https://www.britannica.com/dictionary/%27tis

https://www.merriam-webster.com/dictionary/%27tis

https://dictionary.cambridge.org/us/dictionary/english/tis

[hollerith]

I've only ever seen it spelled 'tis

[dkjaudyeqooe]

It's a contraction of "it is" so " 'tis " is correct.

[jcranmer]

The apostrophe is in the wrong spot, but 'tis is the correct spelling, and the only one I've ever seen.