Trailing comma in multiline configuration

[sobolevn]

Bug Report

Right now these two configurations are different:

[mypy]
plugins =
  mypy_django_plugin.main

And:

[mypy]
plugins =
  mypy_django_plugin.main,

The first one works correctly. The second one fails with:

» mypy server
setup.cfg:138: error: Error importing plugin "": Empty module name  [misc]
Found 1 error in 1 file (errors prevented further checking)

I think that this error message is very confusing. I've spent several minutes tring to figure out why my plugin under test is broken. Turned out I had an extra comma in the settings.

The same happens with other similar settings. This works:

[mypy]
enable_error_code =
  truthy-bool,
  redundant-expr,
  unused-awaitable,
  ignore-without-code

This raises:

[mypy]
enable_error_code =
  truthy-bool,
  redundant-expr,
  unused-awaitable,
  ignore-without-code,

» mypy server
usage: mypy [-h] [-v] [-V] [more options; see below]
            [-m MODULE] [-p PACKAGE] [-c PROGRAM_TEXT] [files ...]
mypy: error: Invalid error code(s): 

Again, I think that this is confusing and can easily be avoided to 0 cost.

I will check pyproject.toml config files. No idea if they are also affected.