Includes several fixes found during testing:
A check to avoid assertion failure in the flex parser
Changes atomic SEXPR to symbols instead of constant strings to avoid spurious error messages for non-standard characters
Changes to options configuration for Flex for performance (as a result, this does not fix check-synth doesn't flush output #9257).
Fix for --stdin-input-per-line to not set incremental by default for non-interactive inputs.
Note that TPTP parsing still uses the ANTLR parser.
Fixes#9258.
Fixes#8374.
Fixes#1862.
Fixes#791.
Fixescvc5/cvc5-wishues#142.
I increased cmake's minimum version in order to use the FindPython module, which allows the build system to specify python versions that are supported. Currently the support is extended to 3.6 through 3.10. I also removed some backward compatible conditions that do not apply anymore provided that cmake's minimum version is guaranteed to be 3.12.
MacOS builds are reenabled in ci.yml.
