TST: Specify HTML file encoding on PY3
 - [x] closes #16525
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16526?src=pr&el=h1) Report
> Merging [#16526](https://codecov.io/gh/pandas-dev/pandas/pull/16526?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ef487d9e474e8052c0f7c6260de5802a950defad?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16526/graphs/tree.svg?token=eZ4WkYLtcO&height=150&width=650&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16526?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16526   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files         161      161           
  Lines       51063    51063           
=======================================
  Hits        46365    46365           
  Misses       4698     4698
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.63% <ø> (ø)` | :arrow_up: |
| #single | `40.15% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16526?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/16526?src=pr&el=footer). Last update [ef487d9...71ebff6](https://codecov.io/gh/pandas-dev/pandas/pull/16526?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/16526?src=pr&el=h1) Report
> Merging [#16526](https://codecov.io/gh/pandas-dev/pandas/pull/16526?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ef487d9e474e8052c0f7c6260de5802a950defad?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16526/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16526?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16526   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files         161      161           
  Lines       51063    51063           
=======================================
  Hits        46365    46365           
  Misses       4698     4698
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.63% <ø> (ø)` | :arrow_up: |
| #single | `40.15% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16526?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/16526?src=pr&el=footer). Last update [ef487d9...71ebff6](https://codecov.io/gh/pandas-dev/pandas/pull/16526?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!