TST: Add pytest-instafail to environment.yml
Pretty useful pytest plugin for getting immediate feedback on test failures instead of waiting for an entire test suite to run:

```
(pandas-dev) ➜  pandas git:(add-pytest-instafail) ✗ pytest pandas/tests/groupby --instafail
============================================================================================ test session starts ============================================================================================
platform darwin -- Python 3.8.5, pytest-6.0.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/danielsaxton/pandas, configfile: setup.cfg
plugins: hypothesis-5.28.0, forked-1.2.0, asyncio-0.12.0, xdist-2.1.0, cov-2.10.1, instafail-0.4.1
collected 4533 items                                                                                                                                                                                        

pandas/tests/groupby/test_allowlist.py .............................................................................................................................................................. [  3%]
...........................................................                                                                                                                                           [  4%]
pandas/tests/groupby/test_apply.py ............x.x.x...................................................x..........................F

________________________________________________________________________________________________ test_fails _________________________________________________________________________________________________

    def test_fails():
>       assert False
E       assert False

pandas/tests/groupby/test_apply.py:1101: AssertionError

pandas/tests/groupby/test_apply_mutate.py ...                                                                                                                                                         [  6%]
pandas/tests/groupby/test_bin_groupby.py ..........                                                                                                                                                   [  7%]
pandas/tests/groupby/test_categorical.py .....................................................................................xxx..............................sss................................x.. [ 10%]
...
```