TST: update tests\plotting\test_frame.py message check for mpl 3.1.0
new error message for df.plot.line

https://travis-ci.org/pandas-dev/pandas/jobs/539282780
TestDataFramePlots.test_subplots is also failing not part of this PR # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26577?src=pr&el=h1) Report
> Merging [#26577](https://codecov.io/gh/pandas-dev/pandas/pull/26577?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a60d1bd45a99519fad5024068db956e0aa1cc6a1?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26577/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26577?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26577      +/-   ##
==========================================
- Coverage   91.84%   91.83%   -0.01%     
==========================================
  Files         174      174              
  Lines       50643    50644       +1     
==========================================
- Hits        46515    46511       -4     
- Misses       4128     4133       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.37% <100%> (ø)` | :arrow_up: |
| #single | `41.69% <100%> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26577?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/26577/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb21wYXQucHk=) | `84.61% <100%> (+1.28%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26577/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26577/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26577/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.81% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26577?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/26577?src=pr&el=footer). Last update [a60d1bd...fff130d](https://codecov.io/gh/pandas-dev/pandas/pull/26577?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @simonjayhawkins 