`pip install -e .` fails on second invocation with "Cython version is too old"

[hayk-skydio]

Describe the bug On a fresh Python environment I'm able to build SymForce with pip install -e ., but calling it again fails.

    running build_ext
    SymEngine_DIR : /Users/hayk/projects/symforce/build/temp.macosx-11.1-arm64-cpython-310/symengine_install/lib/cmake/symengine
    SymEngine Version : 0.7.0
    -- Python include path: /Users/hayk/miniconda3/envs/symforceZ/include/python3.10
    -- Python version: 3.10
    -- Python install path: /Users/hayk/projects/symforce/build/temp.macosx-11.1-arm64-cpython-310/symengine_install/lib/python3.10/site-packages
    -- Found CYTHON: /private/var/folders/sd/jqg3nmsn5ns1yzfp9l970xqh0000gn/T/pip-build-env-vmss160g/normal/bin/cython
    CMake Error at cmake/FindCython.cmake:41 (MESSAGE):
      Your Cython version is too old.  Please upgrade Cython.STDOUT: STDERROR:
    Call Stack (most recent call first):
      CMakeLists.txt:17 (find_package)

    -- Configuring incomplete, errors occurred!

I did naively try pip install Cython which installed Cython-0.29.30, but didn't fix the issue.

To Reproduce Steps to reproduce the behavior, e.g.:

1. conda create --name symforce-test "python>=3.8"
2. conda activate symforce-test
3. pip install -e .
4. pip install -e . again

Environment (please complete the following information):

• OS and version: Mac OS 12.2 M1
• Python version 3.10.4
• SymForce Version 3af09b8, but no changes from main

[aaron-skydio]

Yeah related comments on the wheels PR here, this is due to pip install -e . using a different python environment but not cleaning the symenginepy build

[gcross-zipline]

For those that stumble on this issue and want to work around it: Deleting the build directory resolved this issue for me.

[aaron-skydio]

You should also be able to just delete the directory build/temp.linux-x86_64-cpython-38/symenginepy-prefix/src/symenginepy-build/build/lib.linux-x86_64-cpython-38 within the build directory (replacing linux, x86_64, and/or 38 as necessary) to do less of a rebuild