REF remove unused imports from tseries.frequencies; update imports elsewhere
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=h1) Report
> Merging [#24834](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a07d68582cdc49daf3e6a14a1e1fd7064670d696?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `88.57%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24834/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24834      +/-   ##
==========================================
+ Coverage   92.38%   92.39%   +<.01%     
==========================================
  Files         166      166              
  Lines       52382    52370      -12     
==========================================
- Hits        48394    48385       -9     
+ Misses       3988     3985       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <88.57%> (ø)` | :arrow_up: |
| #single | `42.89% <25.71%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `98.52% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.06% <100%> (ø)` | :arrow_up: |
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `97.7% <100%> (+0.62%)` | :arrow_up: |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `97.23% <100%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.76% <100%> (-0.06%)` | :arrow_down: |
| [pandas/plotting/\_timeseries.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL190aW1lc2VyaWVzLnB5) | `65.46% <60%> (+0.17%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.13% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=footer). Last update [a07d685...5a993da](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=h1) Report
> Merging [#24834](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a07d68582cdc49daf3e6a14a1e1fd7064670d696?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `88.57%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24834/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24834      +/-   ##
==========================================
+ Coverage   92.38%   92.39%   +<.01%     
==========================================
  Files         166      166              
  Lines       52382    52370      -12     
==========================================
- Hits        48394    48385       -9     
+ Misses       3988     3985       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <88.57%> (ø)` | :arrow_up: |
| #single | `42.89% <25.71%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `98.52% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.06% <100%> (ø)` | :arrow_up: |
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `97.7% <100%> (+0.62%)` | :arrow_up: |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `97.23% <100%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.76% <100%> (-0.06%)` | :arrow_down: |
| [pandas/plotting/\_timeseries.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL190aW1lc2VyaWVzLnB5) | `65.46% <60%> (+0.17%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24834/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.13% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=footer). Last update [a07d685...5a993da](https://codecov.io/gh/pandas-dev/pandas/pull/24834?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!