solve list cannot add to tuple error in make_signature function
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13878?src=pr) is 85.23% (diff: 0.00%)

> Merging [#13878](https://codecov.io/gh/pydata/pandas/pull/13878?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

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  Hits          43007      43007          
  Misses         7449       7449          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [d4f95fd...1ecdeb6](https://codecov.io/gh/pydata/pandas/compare/d4f95fdebb74db5edb09c743b48c4b03e0940591...1ecdeb6e2c8d9fd8ffa8bf3259184ad3b4a90f02?src=pr)
 what exactly does this break?
 when I run an example like: 

from pandas.util.decorators import make_signature
def f(a,b,c=2) :
    return a_b_c

print(make_signature(f))

I got an error as: 
defaults = ('',) \* n_wo_defaults + spec.defaults
TypeError: can only concatenate tuple (not "list") to tuple
 hmm, that does fail (only in py3). Well must have been a typo in the copy paste a while back. Can you add a test in `pandas/tests/test_util.py`.  `make_signature` is actually used in `pandas.core.groupby`. Can you repro an error from something?
 can you rebase / update?
