3.18.0 isn't compatible with Python 2 despite being installable

[convoliution]

What version of protobuf and what language are you using?
Version: v3.18.0
Language: Python

What operating system (Linux, Windows, ...) and version?
macOS Mojave 10.14.6

What runtime / compiler are you using (e.g., python version or gcc version)

Python 2.7.18 (default, Mar 30 2021, 14:20:09) 
[GCC 4.2.1 Compatible Apple LLVM 10.0.1 (clang-1001.0.46.4)] on darwin

What did you do?
Steps to reproduce the behavior:

1. Activate a Python 2 environment
2. pip install protobuf==3.18.0
3. python -c "from google.protobuf import descriptor"

What did you expect to see
Either:

• ERROR: Could not find a version that satisfies the requirement protobuf==3.18.0
  in (2)
• no errors in (3)

What did you see instead?

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/convoliution/.pyenv/versions/dev2/lib/python2.7/site-packages/google/protobuf/descriptor.py", line 113
    class DescriptorBase(metaclass=DescriptorMetaclass):
                                  ^
SyntaxError: invalid syntax

in (3)

[srenauld]

This breaks nrfutil, rendering a major provider's CLI tool for DFU upgrades inoperable with no way to pin this dependency.

[odelrio]

https://github.com/googleapis/python-api-core depends on protobuf and this version breaks Python2.7-based builds. My workaround was to force v3.17.3 by defining a new dependency.

[busunkim96]

@odelrio I just released a 1.x version of google-api-core that pins to protobuf <3.18.0. https://pypi.org/project/google-api-core/1.31.3/

[alanyee]

#9045