Add py39 target in Black's configuration, bump Black to 21.5b2

[fangchenli]

No description provided.

[fangchenli]

The py39 option is only on Black's master. Close for now.

[MarcoGorelli]

Thanks @fangchenli - check #41160, there's some comments there about some things which probably should be manually fixed up

[jreback]

can you rebase

[jreback]

cc @MarcoGorelli are we generally ok with this?

[MarcoGorelli]

Looks good to me, seems that you made these changes manually:

$ git diff HEAD
diff --git a/doc/source/development/contributing_codebase.rst b/doc/source/development/contributing_codebase.rst
index 411b65fdc9..d6ff48ed5f 100644
--- a/doc/source/development/contributing_codebase.rst
+++ b/doc/source/development/contributing_codebase.rst
@@ -169,7 +169,7 @@ submitting code to run the check yourself::
 to auto-format your code. Additionally, many editors have plugins that will
 apply ``black`` as you edit files.
 
-You should use a ``black`` version 21.5b0 as previous versions are not compatible
+You should use a ``black`` version 20.8b1 as previous versions are not compatible
 with the pandas codebase.
 
 One caveat about ``git diff upstream/master -u -- "*.py" | flake8 --diff``: this
diff --git a/environment.yml b/environment.yml
index 332b853272..bb96235123 100644
--- a/environment.yml
+++ b/environment.yml
@@ -18,7 +18,7 @@ dependencies:
   - cython>=0.29.21
 
   # code checks
-  - black=21.5b0
+  - black=20.8b1
   - cpplint
   - flake8=3.9.2
   - flake8-bugbear=21.3.2  # used by flake8, find likely bugs
diff --git a/pandas/_libs/algos.pyi b/pandas/_libs/algos.pyi
index 96dfd81153..a2d028250e 100644
--- a/pandas/_libs/algos.pyi
+++ b/pandas/_libs/algos.pyi
@@ -50,15 +50,15 @@ def nancorr(
     mat: np.ndarray,  # const float64_t[:, :]
     cov: bool = False,
     minp=None,
-) -> np.ndarray: ...  # ndarray[float64_t, ndim=2]
+) -> np.ndarray: ...
 def nancorr_spearman(
     mat: np.ndarray,  # ndarray[float64_t, ndim=2]
     minp: int = 1,
-) -> np.ndarray: ...  # ndarray[float64_t, ndim=2]
+) -> np.ndarray: ...
 def nancorr_kendall(
     mat: np.ndarray,  # ndarray[float64_t, ndim=2]
     minp: int = 1,
-) -> np.ndarray: ...  # ndarray[float64_t, ndim=2]
+) -> np.ndarray: ...
 
 # ----------------------------------------------------------------------
 
diff --git a/pandas/_libs/tslibs/conversion.pyi b/pandas/_libs/tslibs/conversion.pyi
index e74a56a519..10c8a819b9 100644
--- a/pandas/_libs/tslibs/conversion.pyi
+++ b/pandas/_libs/tslibs/conversion.pyi
@@ -12,7 +12,7 @@ class OutOfBoundsTimedelta(ValueError): ...
 
 def precision_from_unit(
     unit: str,
-) -> tuple[int, int,]: ...  # (int64_t, _)
+) -> tuple[int, int,]: ...  # int64_t
 def ensure_datetime64ns(
     arr: np.ndarray,  # np.ndarray[datetime64[ANY]]
     copy: bool = ...,
@@ -23,5 +23,5 @@ def ensure_timedelta64ns(
 ) -> np.ndarray: ...  # np.ndarray[timedelta64ns]
 def datetime_to_datetime64(
     values: np.ndarray,  # np.ndarray[object]
-) -> tuple[np.ndarray, tzinfo | None,]: ...  # (np.ndarray[dt64ns], _)
+) -> tuple[np.ndarray, tzinfo | None,]: ...  # np.ndarray[dt64ns]
 def localize_pydatetime(dt: datetime, tz: tzinfo | None) -> datetime: ...
diff --git a/pandas/_libs/window/indexers.pyi b/pandas/_libs/window/indexers.pyi
index f65deae6fd..2dea936222 100644
--- a/pandas/_libs/window/indexers.pyi
+++ b/pandas/_libs/window/indexers.pyi
@@ -7,7 +7,4 @@ def calculate_variable_window_bounds(
     center: bool,
     closed: str | None,
     index: np.ndarray,  # const int64_t[:]
-) -> tuple[
-    np.ndarray,
-    np.ndarray,
-]: ...  # (np.ndarray[np.int64]  # np.ndarray[np.int64])
+) -> tuple[np.ndarray, np.ndarray,]: ...  # np.ndarray[np.int64]  # np.ndarray[np.int64]
diff --git a/pyproject.toml b/pyproject.toml
index 3947856d94..01d28777eb 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -14,7 +14,7 @@ requires = [
 # build-backend = "setuptools.build_meta"
 
 [tool.black]
-target-version = ['py37', 'py38', 'py39']
+target-version = ['py37', 'py38']
 exclude = '''
 (
     asv_bench/env
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 7f07c499e1..f454bfd152 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -6,7 +6,7 @@ python-dateutil>=2.7.3
 pytz
 asv
 cython>=0.29.21
-black==21.5b0
+black==20.8b1
 cpplint
 flake8==3.9.2
 flake8-bugbear==21.3.2

which I believe address @jbrockmendel 's comments on the other PR

[jbrockmendel]

same as above re this being multiple lines

[jbrockmendel]

this is ambiguous which ndarray the comment refers to

[jbrockmendel]

this case isnt ambiguous, but it is less obvious

[jbrockmendel]

havent seen this pattern elsewhere?

[fangchenli]

can we use this pattern # (comment for A, comment for B) for the return type of tuple[A, B]? It's more explicit than the one reformated by black

[simonjayhawkins]

Thanks @fangchenli can we do this before 1.3.0rc0 or after 1.3.0. updating master during the rc period will potentially make auto backports fail.

[fangchenli]

Thanks @fangchenli can we do this before 1.3.0rc0 or after 1.3.0. updating master during the rc period will potentially make auto backports fail.

I'll try to finish this before 1.3.0rc0.

[jreback]

consensus is to do this before the RC, @fangchenli can you merge master

[jreback]

thanks @fangchenli (we might need to add this to the development changes in whatsnew but can followup on that)