mitogen.parent: Eliminate use of platform module in first stage

This reduces the size of the initial SSH command by 204 bytes, & may fix errors
running Mitogen on  macOS. AFAICT platform was used but not imported.

Before
```
$ python ./preamble_size.py
SSH command size: 833
Bootstrap (mitogen.core) size: 17007 (16.61KiB)

                              Original          Minimized           Compressed
mitogen.parent            97565 95.3KiB  50427 49.2KiB 51.7%  12689 12.4KiB 13.0%
mitogen.fork               8436  8.2KiB   4130  4.0KiB 49.0%   1648  1.6KiB 19.5%
mitogen.ssh               10892 10.6KiB   6952  6.8KiB 63.8%   2113  2.1KiB 19.4%
mitogen.sudo              12089 11.8KiB   5924  5.8KiB 49.0%   2249  2.2KiB 18.6%
mitogen.select            12325 12.0KiB   2929  2.9KiB 23.8%    964  0.9KiB 7.8%
mitogen.service           41644 40.7KiB  22431 21.9KiB 53.9%   5886  5.7KiB 14.1%
mitogen.fakessh           15599 15.2KiB   8011  7.8KiB 51.4%   2624  2.6KiB 16.8%
mitogen.master            48732 47.6KiB  24569 24.0KiB 50.4%   6768  6.6KiB 13.9%
```

After
```
$ python preamble_size.py
SSH command size: 629
Bootstrap (mitogen.core) size: 17007 (16.61KiB)

                              Original          Minimized           Compressed
mitogen.parent            97543 95.3KiB  50357 49.2KiB 51.6%  12665 12.4KiB 13.0%
mitogen.fork               8436  8.2KiB   4130  4.0KiB 49.0%   1648  1.6KiB 19.5%
mitogen.ssh               10892 10.6KiB   6952  6.8KiB 63.8%   2113  2.1KiB 19.4%
mitogen.sudo              12089 11.8KiB   5924  5.8KiB 49.0%   2249  2.2KiB 18.6%
mitogen.select            12325 12.0KiB   2929  2.9KiB 23.8%    964  0.9KiB 7.8%
mitogen.service           41644 40.7KiB  22431 21.9KiB 53.9%   5886  5.7KiB 14.1%
mitogen.fakessh           15599 15.2KiB   8011  7.8KiB 51.4%   2624  2.6KiB 16.8%
mitogen.master            48732 47.6KiB  24569 24.0KiB 50.4%   6768  6.6KiB 13.9%
```

docs/changelog.rst

@@ -25,6 +25,7 @@ v0.3.1.dev0 (unreleased)
 * :gh:issue:`869` Continuous Integration tests are now run with Tox
 * :gh:issue:`869` Continuous Integration tests now cover CentOS 6 & 8, Debian 9 & 11, Ubuntu 16.04 & 20.04
 * :gh:issue:`860` Add initial support for podman connection (w/o Ansible support yet)
+* :gh:issue:`873` `python -c ...` first stage no longer uses :py:mod:`platform`` to detect the macOS release
 
 
 v0.3.0 (2021-11-24)

mitogen/parent.py

@@ -42,7 +42,6 @@ import heapq
 import inspect
 import logging
 import os
-import platform
 import re
 import signal
 import socket
@@ -1410,9 +1409,12 @@ class Connection(object):
     #     their respective values.
     #   * CONTEXT_NAME must be prefixed with the name of the Python binary in
     #     order to allow virtualenvs to detect their install prefix.
-    #   * For Darwin, OS X installs a craptacular argv0-introspecting Python
-    #     version switcher as /usr/bin/python. Override attempts to call it
-    #     with an explicit call to python2.7
+    #   * macOS <= 10.14 (Darwin <= 18) install an unreliable Python version
+    #     switcher as /usr/bin/python, which introspects argv0. To workaround
+    #     it we redirect attempts to call /usr/bin/python with an explicit
+    #     call to /usr/bin/python2.7. macOS 10.15+ (Darwin 19+) removed it.
+    #     On these versions /usr/bin/python is a symlink to
+    #     /System/Library/Frameworks/Python.framework/Versions/2.7/.../Python
     #
     # Locals:
     #   R: read side of interpreter stdin.
@@ -1435,11 +1437,7 @@ class Connection(object):
             os.close(r)
             os.close(W)
             os.close(w)
-            # this doesn't apply anymore to Mac OSX 10.15+ (Darwin 19+), new interpreter looks like this:
-            # /System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python
-            if sys.platform == 'darwin' and sys.executable == '/usr/bin/python' and \
-                    int(platform.release()[:2]) < 19:
-                sys.executable += sys.version[:3]
+            if sys.executable+sys.platform=='/usr/bin/pythondarwin'and os.uname()[2]<'19':sys.executable+='2.7'
             os.environ['ARGV0']=sys.executable
             os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
         os.write(1,'MITO000\n'.encode())