Python 3.10 breaks build

[duanqn]

FindPython.cmake assumes python minor version is single digit.

[duanqn]

Another reason: pyreadline/pyreadline#73

[sarcXD]

@duanqn Found this in another issue thread, figured this was the correct place to post this. Firstly, glad I'm not the only one coming across this, almost drove me nuts. Luckily I found a relatively clean and arguably better and cleaner way of setting our ${PYTHON_MIN_VERSION_HEX} variable. I'll outline the relatively simple fix here.
Firstly, this is how cpython does it in their header library

#define PY_VERSION_HEX ((PY_MAJOR_VERSION << 24) | \
                        (PY_MINOR_VERSION << 16) | \
                        (PY_MICRO_VERSION <<  8) | \
                        (PY_RELEASE_LEVEL <<  4) | \
                        (PY_RELEASE_SERIAL << 0))

Now, All we need to do is shift the major version up by 24 bits, and minor version up by 16
Current method:

set(PYTHON_MIN_VERSION_HEX "0x0${Python3_VERSION_MAJOR}0${Python3_VERSION_MINOR}0000")

The fix I made (making, will send a PR after this)

math(EXPR BIT_SHIFT_HEX "${Python3_VERSION_MAJOR} << 24 | ${Python3_VERSION_MINOR} << 16" OUTPUT_FORMAT HEXADECIMAL)
set(PYTHON_MIN_VERSION_HEX "${BIT_SHIFT_HEX}")

see cmake page for math that allows c like bit manipulation operations
That fixes it basically.

[duanqn]

@sarcXD This looks great!
We still have issues with pyreadline, but fixing the hex version is definitely a great improvement.