Fix validation error type `RT04` and check in CI

[kynan]

Problem description

In order to have a continuous check by the CI on validation error RT04 (Return value description should start with a capital letter), fixing them in the code base enables the addition to the CI for automated testing in the future.

RT04 has just 33 occurrences (see overview of the errors):

method	file	file_line	github_link
pandas.core.resample.Resampler.apply	pandas/core/resample.py	258.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/resample.py#L258
pandas.core.resample.Resampler.aggregate	pandas/core/resample.py	258.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/resample.py#L258
pandas.Series.agg	pandas/core/series.py	3427.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L3427
pandas.Series.aggregate	pandas/core/series.py	3427.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L3427
pandas.Series.quantile	pandas/core/series.py	2004.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L2004
pandas.Series.asof	pandas/core/generic.py	6855.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6855
pandas.Series.dt.to_pydatetime	pandas/core/indexes/accessors.py	129.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/accessors.py#L129
pandas.Series.dt.to_pytimedelta	pandas/core/indexes/accessors.py	199.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/accessors.py#L199
pandas.Index.shift	pandas/core/indexes/base.py	4242.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/base.py#L4242
pandas.MultiIndex.get_level_values	pandas/core/indexes/multi.py	1380.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/multi.py#L1380
pandas.DataFrame.agg	pandas/core/frame.py	6295.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L6295
pandas.DataFrame.aggregate	pandas/core/frame.py	6295.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L6295
pandas.DataFrame.quantile	pandas/core/frame.py	7717.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/frame.py#L7717
pandas.DataFrame.asof	pandas/core/generic.py	6855.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6855
pandas.Panel.major_xs	pandas/core/panel.py	793.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/panel.py#L793
pandas.Panel.minor_xs	pandas/core/panel.py	817.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/panel.py#L817
pandas.api.types.is_categorical_dtype	pandas/core/dtypes/common.py	572.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L572
pandas.api.types.is_datetime64_dtype	pandas/core/dtypes/common.py	403.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L403
pandas.api.types.is_datetime64tz_dtype	pandas/core/dtypes/common.py	434.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L434
pandas.api.types.is_extension_type	pandas/core/dtypes/common.py	1643.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L1643
pandas.api.types.is_integer_dtype	pandas/core/dtypes/common.py	868.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L868
pandas.api.types.is_interval_dtype	pandas/core/dtypes/common.py	536.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L536
pandas.api.types.is_signed_integer_dtype	pandas/core/dtypes/common.py	923.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L923
pandas.api.types.is_timedelta64_dtype	pandas/core/dtypes/common.py	472.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L472
pandas.api.types.is_timedelta64_ns_dtype	pandas/core/dtypes/common.py	1167.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L1167
pandas.api.types.is_unsigned_integer_dtype	pandas/core/dtypes/common.py	980.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L980
pandas.api.types.is_datetimetz	pandas/core/dtypes/common.py	294.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/common.py#L294
pandas.unique	pandas/core/algorithms.py	278.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/algorithms.py#L278
pandas.core.window.Rolling.aggregate	pandas/core/window.py	1678.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/window.py#L1678
pandas.core.window.Expanding.aggregate	pandas/core/window.py	1956.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/window.py#L1956
pandas.io.formats.style.Styler.from_custom_template	pandas/io/formats/style.py	1210.0	https://github.com/pandas-dev/pandas/blob/master/pandas/io/formats/style.py#L1210
pandas.io.formats.style.Styler.render	pandas/io/formats/style.py	421.0	https://github.com/pandas-dev/pandas/blob/master/pandas/io/formats/style.py#L421
pandas.Panel.asof	pandas/core/generic.py	6855.0	https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L6855

Todo:

• get rid of the errors in the code base
• update the code_check.sh script to take into account the RT04 type of errors

[MarckK]

Will have a try on this.