mitogen.parent: Detect and avoid Python2.7 wrapper on macOS 11 & 12

Without this errors such as the following occur
```
✗ MITOGEN_LOG_LEVEL=DEBUG python3 foo.py
Python: execv: (null): No such file or directory
Traceback (most recent call last):
  File "foo.py", line 16, in <module>
    target = router.local(python_path='/usr/bin/python2.7', debug=True)
  File "/Users/alex/src/mitogen2/mitogen/parent.py", line 2486, in local
    return self.connect(u'local', **kwargs)
  File "/Users/alex/src/mitogen2/mitogen/parent.py", line 2446, in connect
    return self._connect(klass, **mitogen.core.Kwargs(kwargs))
  File "/Users/alex/src/mitogen2/mitogen/parent.py", line 2426, in _connect
    conn.connect(context=context)
  File "/Users/alex/src/mitogen2/mitogen/parent.py", line 1708, in connect
    raise self.exception
mitogen.parent.EofError: EOF on stream; last 100 lines received:
MITO000
MITO001
```

Before
```
$ ./preamble_size.py
SSH command size: 625
Bootstrap (mitogen.core) size: 17007 (16.61KiB)

                              Original          Minimized           Compressed
mitogen.parent            97496 95.2KiB  50355 49.2KiB 51.6%  12663 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
```
$ ./preamble_size.py
SSH command size: 705
Bootstrap (mitogen.core) size: 17007 (16.61KiB)

                              Original          Minimized           Compressed
mitogen.parent            97885 95.6KiB  50516 49.3KiB 51.6%  12728 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            48733 47.6KiB  24570 24.0KiB 50.4%   6771  6.6KiB 13.9%
```

Author: moreati

docs/changelog.rst

@@ -21,6 +21,7 @@ To avail of fixes in an unreleased version, please download a ZIP file
 v0.3.1.dev0 (unreleased)
 ------------------------
 
+* :gh:issue:`774` Fix bootstrap failures on macOS 11.x and 12.x, involving Python 2.7 wrapper
 * :gh:issue:`834` Support for Ansible 3 and 4 (ansible-core 2.11)
 * :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

mitogen/parent.py

@@ -1412,9 +1412,12 @@ def __repr__(self):
     #   * 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
+    #     call to /usr/bin/python2.7. macOS 10.15 (Darwin 19) removed it.
+    #   * macOS 11.x (Darwin 20, Big Sur) and macOS 12.x (Darwin 21, Montery)
+    #     do something slightly different. The Python executable is patched to
+    #     perform an extra execvp(). I don't fully understand the details, but
+    #     setting PYTHON_LAUNCHED_FROM_WRAPPER=1 avoids it.
+    #   * macOS 13.x (Darwin 22?) may remove python 2.x entirely.
     #
     # Locals:
     #   R: read side of interpreter stdin.
@@ -1437,7 +1440,8 @@ def _first_stage():
             os.close(r)
             os.close(W)
             os.close(w)
-            if sys.executable+sys.platform=='/usr/bin/pythondarwin'and os.uname()[2]<'19':sys.executable+='2.7'
+            if os.uname()[0]=='Darwin'and os.uname()[2][:2]<'19'and sys.executable=='/usr/bin/python':sys.executable='/usr/bin/python2.7'
+            if os.uname()[0]=='Darwin'and os.uname()[2][:2]in'2021'and sys.version[:3]=='2.7':os.environ['PYTHON_LAUNCHED_FROM_WRAPPER']='1'
             os.environ['ARGV0']=sys.executable
             os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
         os.write(1,'MITO000\n'.encode())