Add get_generic_param() and fix finding Returns[]

[wRAR]

Fixes #182 and will help in https://github.com/zytedata/zyte-common-items/issues/49

[codecov[bot]]

Codecov Report

Merging #184 (54d4948) into master (ffaa303) will decrease coverage by 0.14%. Report is 1 commits behind head on master. The diff coverage is 93.75%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #184 +/- ## ========================================== - Coverage 98.54% 98.40% -0.14% ========================================== Files 30 29 -1 Lines 1378 1383 +5 ========================================== + Hits 1358 1361 +3 - Misses 20 22 +2 ``` | [Files Changed](https://app.codecov.io/gh/scrapinghub/web-poet/pull/184?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub) | Coverage Δ | | |---|---|---| | [web\_poet/pages.py](https://app.codecov.io/gh/scrapinghub/web-poet/pull/184?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-d2ViX3BvZXQvcGFnZXMucHk=) | `96.25% <86.66%> (-2.37%)` | :arrow_down: | | [web\_poet/rules.py](https://app.codecov.io/gh/scrapinghub/web-poet/pull/184?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-d2ViX3BvZXQvcnVsZXMucHk=) | `100.00% <100.00%> (ø)` | | | [web\_poet/utils.py](https://app.codecov.io/gh/scrapinghub/web-poet/pull/184?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-d2ViX3BvZXQvdXRpbHMucHk=) | `99.27% <100.00%> (+0.08%)` | :arrow_up: |

[kmike]

Let's merge it; I don't think typing discussion is important enough.

[kmike]

Thanks @wRAR!