[requirements.txt] change fontTools version from '<=' to '=='

[kaydeearts]

to accept more versions.
Should fix #1660

Description

• currently, setup.py changes '==' to '>=' but we had set fontTools to '<=' due to an earlier issue where we had to pin the version number. We no longer need to pin the version number, so we're changing it back to '=='.
• as Miguel mentioned, we should add support for more string replacement cases in setup.py — I'll look at that in a few days, but pushing this PR first to get a pre-release out for issue AFDKO 3.9.5 incompatible with FontTools > 4.39.3. Any plans to support FontTools 4.39.4? #1660 .

Checklist:

• I have followed the Contribution Guidelines
• I have added test code and data to prove that my code functions correctly
• I have verified that new and existing tests pass locally with my changes
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation