Somehow, it turns out that `TCP_NOTSENT_LOWAT` that's available since 3.7.x is not available in the official macOS builds 🙀:

> python3.7
Python 3.7.4 (v3.7.4:e09359112e, Jul  8 2019, 14:54:52)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> socket.TCP_NOTSENT_LOWAT
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'socket' has no attribute 'TCP_NOTSENT_LOWAT'

> python3.8
Python 3.8.2 (v3.8.2:7b3ab5921f, Feb 24 2020, 17:52:18)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> socket.TCP_NOTSENT_LOWAT
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'socket' has no attribute 'TCP_NOTSENT_LOWAT'

> python3.9
Python 3.9.0a4 (v3.9.0a4:6e02691f30, Feb 25 2020, 18:14:13)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> socket.TCP_NOTSENT_LOWAT
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'socket' has no attribute 'TCP_NOTSENT_LOWAT'

And my local build has it 😺:

> ~/cpython/python.exe
Python 3.9.0a4+ (heads/master:be501ca241, Mar  4 2020, 15:16:49)
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> socket.TCP_NOTSENT_LOWAT
513

So... my guess is official builds are using old SDK or header files. 🤔

My system has it e.g. here:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/netinet/tcp.h
230:#define TCP_NOTSENT_LOWAT       0x201   /* Low water mark for TCP unsent data */

And in fact it's present in every `netinet/tcp.h` on my system: CommandLineTools 10.14 and 10.5 sdks; MacOSX dev sdk, {AppleTV,Watch,iPhone}{OS,Simulator} sdks.