Set better headers for HTTP requests

[jwodder]

• Set User-Agent for all requests
• Set Accept and X-GitHub-Api-Version for GitHub requests to their recommended values

[codecov-commenter]

Codecov Report

Attention: 24 lines in your changes are missing coverage. Please review.

Comparison is base (aab966e) 64.80% compared to head (b98703b) 64.63%. Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #178 +/- ## ========================================== - Coverage 64.80% 64.63% -0.17% ========================================== Files 1 1 Lines 1500 1513 +13 Branches 346 346 ========================================== + Hits 972 978 +6 - Misses 431 438 +7 Partials 97 97 ``` | [Files](https://app.codecov.io/gh/datalad/datalad-installer/pull/178?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | Coverage Δ | | |---|---|---| | [src/datalad\_installer.py](https://app.codecov.io/gh/datalad/datalad-installer/pull/178?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-c3JjL2RhdGFsYWRfaW5zdGFsbGVyLnB5) | `64.63% <33.33%> (-0.17%)` | :arrow_down: |

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[yarikoptic]

it is a last PR in a series ATM, I marked it to cut release. Thank you @jwodder !