Bug#1028308: mozjs78: FTBFS with python 3.11

[Vincent Cheng]

Source: mozjs78
Version: 78.15.0-5
Severity: serious
Tags: ftbfs

mozjs78 FTBFS since Python 3.11 became the default Python3 version in
sid. The relevant part of the build log is pasted below.

Incidentally, src:0ad happens to FTBFS for the same reason because it
embeds mozjs/spidermonkey [1]. You can see how we fixed this in 0ad
(unfortunately this involves cherry-picking an upstream commit to
upgrade virtualenv) [2].

```
checking for valloc in malloc.h... yes
checking for valloc in unistd.h... no
checking for _aligned_malloc in malloc.h... no
updating cache ./config.cache
creating ./config.data
Creating config.status
Traceback (most recent call last):
  File "/build/mozjs78-78.15.0/js/src/../../configure.py", line 181, in <module>
    sys.exit(main(sys.argv))
             ^^^^^^^^^^^^^^
  File "/build/mozjs78-78.15.0/js/src/../../configure.py", line 57, in main
    return config_status(config)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/build/mozjs78-78.15.0/js/src/../../configure.py", line 142,
in config_status
    partial_config.write_vars(sanitized_config)
  File "/build/mozjs78-78.15.0/python/mozbuild/mozbuild/backend/configenvironment.py",
line 361, in write_vars
    self.substs._fill_group(substs)
  File "/build/mozjs78-78.15.0/python/mozbuild/mozbuild/backend/configenvironment.py",
line 257, in _fill_group
    new_files.add(self._write_file(k, v))
                  ^^^^^^^^^^^^^^^^^^^^^^
  File "/build/mozjs78-78.15.0/python/mozbuild/mozbuild/backend/configenvironment.py",
line 240, in _write_file
    with FileAvoidWrite(filename) as fh:
  File "/build/mozjs78-78.15.0/python/mozbuild/mozbuild/util.py", line
338, in __exit__
    self.close()
  File "/build/mozjs78-78.15.0/python/mozbuild/mozbuild/util.py", line
261, in close
    existing = _open(self.name, self.mode)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/build/mozjs78-78.15.0/python/mozbuild/mozbuild/util.py", line
59, in _open
    return io.open(path, mode, encoding='utf-8', newline='\n')
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid mode: 'rU'
Configure failed with status 1
```

Regards,
Vincent

[1] https://bugs.debian.org/1028179
[2] https://salsa.debian.org/games-team/0ad/-/commit/7048ef33282782d9af46335c9d928dfa9d9f379d