this breaks builds for ubuntu, I'd suggest reverting this (especially because it appears to build fine without this patch)

2020-06-29T08:52:56.8303672Z x86_64-linux-gnu-gcc -pthread -fPIC -Wdate-time -D_FORTIFY_SOURCE=2 -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -g -fdebug-prefix-map=/tmp/code=. -specs=/usr/share/dpkg/no-pie-compile.specs -fstack-protector -Wformat -Werror=format-security -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -fvisibility=hidden -I../Include/internal -DCONFIG_64=1 -DASM=1 -I../Include -IObjects -IPython -I. -I/usr/include/x86_64-linux-gnu -I/tmp/code/Include -I/tmp/code/build-static -c /tmp/code/Modules/_decimal/_decimal.c -o build/temp.linux-x86_64-3.9/tmp/code/Modules/_decimal/_decimal.o
2020-06-29T08:52:56.9003112Z /tmp/code/Modules/_decimal/_decimal.c:40:4: error: #error "libmpdec version >= 2.5.0 required"
2020-06-29T08:52:56.9005053Z    #error "libmpdec version >= 2.5.0 required"
2020-06-29T08:52:56.9006190Z     ^~~~~