Curly braces cannot be escaped in env substitution

[blueyed]

commands = echo {env:DOESNOTEXIST:@\{u\}} results in: {env:DOESNOTEXIST:@{u}}.

Without escaping them (commands = echo {env:DOESNOTEXIST:@{u}}):

  File "/usr/lib/python3.8/site-packages/tox/config/__init__.py", line 1638, in _substitute_from_other_section
    raise tox.exception.ConfigError("substitution key {!r} not found".format(key))
tox.exception.ConfigError: ConfigError: substitution key 'u' not found

tox 3.14.3

[gaborbernat]

Feel free to open a PR against master and fix it. My available efforts at the moment are aimed at fixing this as part of #1394, but that probably will take a while (ETA September).

[jayvdb]

I've checked most versions back to 2.0 and this never worked.