CLN upgrade to Python3.7+ syntax

[MarcoGorelli]

Given that the next release will we Python3.7+, we might as well upgrade some of the syntax, e.g.

-    for make_index_func in make_index_funcs:
-        yield make_index_func
+    yield from make_index_funcs

or

-            return "%s[%s, %s]" % (cls, tp_repr, metadata_reprs)
+            return f"{cls}[{tp_repr}, {metadata_reprs}]"

We can do this using pyupgrade.

Here are the files that can be upgraded:

• pandas/_config/display.py
• pandas/_testing.py
• pandas/_vendored/typing_extensions.py
• pandas/_version.py
• pandas/core/arrays/datetimes.py
• pandas/core/arrays/sparse/array.py
• pandas/core/common.py
• pandas/core/computation/expr.py
• pandas/core/computation/pytables.py
• pandas/core/generic.py
• pandas/core/groupby/base.py
• pandas/core/groupby/generic.py
• pandas/core/indexes/base.py
• pandas/core/indexes/datetimelike.py
• pandas/core/reshape/merge.py
• pandas/io/clipboard/init.py
• pandas/io/formats/excel.py
• pandas/io/html.py
• pandas/io/json/_json.py
• pandas/io/pytables.py
• pandas/io/stata.py
• pandas/plotting/_matplotlib/core.py
• pandas/tests/arrays/categorical/test_constructors.py
• pandas/tests/arrays/integer/test_construction.py
• pandas/tests/extension/decimal/array.py
• pandas/tests/extension/test_categorical.py
• pandas/tests/frame/test_constructors.py
• pandas/tests/groupby/test_groupby.py
• pandas/tests/groupby/test_grouping.py
• pandas/tests/indexes/test_base.py
• pandas/tests/indexing/test_indexing.py
• pandas/tests/io/formats/test_format.py
• pandas/tests/io/formats/test_to_csv.py
• pandas/tests/io/formats/test_to_html.py
• pandas/tests/io/formats/test_to_latex.py
• pandas/tests/io/json/test_ujson.py
• pandas/tests/io/parser/test_c_parser_only.py
• pandas/tests/io/parser/test_common.py
• pandas/tests/io/parser/test_encoding.py
• pandas/tests/io/parser/test_read_fwf.py
• pandas/tests/io/pytables/common.py
• pandas/tests/io/test_common.py
• pandas/tests/io/test_html.py
• pandas/tests/io/test_sql.py
• pandas/tests/reshape/test_get_dummies.py
• pandas/tests/scalar/test_na_scalar.py
• pandas/tests/series/test_analytics.py
• pandas/tests/series/test_constructors.py
• pandas/tests/series/test_dtypes.py
• pandas/tests/series/test_io.py
• scripts/validate_rst_title_capitalization.py
• scripts/validate_unwanted_patterns.py
• setup.py
• versioneer.py

If you're intereseted in helping out, please choose a batch of 10-15 to run pyupgrade on.

Install pyupgrade with:

pip install pyupgrade==2.7.2

Then, run

pyupgrade <files you want to upgrade> --py37-plus

e.g.

pyupgrade pandas/tests/io/test_html.py pandas/tests/io/json/test_ujson.py --py37-plus

See #36453 for an example PR.

Once this is done, we can add it to pre-commit / CI.

[ahgamut]

I'll try to submit a PR for this. take

[ahgamut]

I've submitted #36457 for all the unchecked files.
It passes all CI checks but one (travis-ci build timeout).