This enables *default* Python mechanism for integer string
conversion length limitation:
https://docs.python.org/3/library/stdtypes.html#integer-string-conversion-length-limitation
For the mpmath CLI it will be *off* by default. Usually, it doesn't
matter, as these limits not affect the gmpy2/gmp backends and working
with mpmath's types (mpf/mpc). Though, sometimes you want to play with
integers in the mpmath console and these limits are really annoying.
I consider this as a bugfix, despite it adds a new option.
Some systems (e.g., Debian/Ubuntu) ship a "python3" command instead of
"python" per PEP 394. But we may not be able to guarantee if
"python3" exists either, so we use sys.executable to determine the
name of the command.