CI: old matplotlib being pulled (again)

[MarcoGorelli]

Another day, another CI failure

Here's the difference between a recent green and red run:

                                                  Name Version_x Version_y
0    ──────────────────────────────────────────────...      None      None
17                                           aws-c-cal    0.5.20    0.5.11
18                                        aws-c-common     0.8.5     0.6.2
19                                  aws-c-event-stream    0.2.15     0.2.7
20                                            aws-c-io   0.13.11    0.10.5
21                                       aws-checksums    0.1.13    0.1.11
23                                         aws-sdk-cpp   1.9.379   1.8.186
221                                         matplotlib     3.3.2     3.6.2
222                                    matplotlib-base     3.3.2     3.6.2
258                                            openssl     3.0.7    1.1.1s
334                                                s2n    1.3.30    1.0.10