FEAT: Refactor to src-layout

[MementoRC]

Proposing a src-layout style. This helps circumvent renaming the coincurve directory during tests. Eventually, when creating the wheel this directory will also remain pristine

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.68%. Comparing base (dba5cbd) to head (0afdb56).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #153 +/- ## ======================================= Coverage 85.68% 85.68% ======================================= Files 11 11 Lines 566 566 Branches 67 67 ======================================= Hits 485 485 Misses 45 45 Partials 36 36 ``` | [Files](https://app.codecov.io/gh/ofek/coincurve/pull/153?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ofek+Lev) | Coverage Δ | | |---|---|---| | [src/coincurve/context.py](https://app.codecov.io/gh/ofek/coincurve/pull/153?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ofek+Lev#diff-c3JjL2NvaW5jdXJ2ZS9jb250ZXh0LnB5) | `80.00% <ø> (ø)` | | | [src/coincurve/ecdsa.py](https://app.codecov.io/gh/ofek/coincurve/pull/153?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ofek+Lev#diff-c3JjL2NvaW5jdXJ2ZS9lY2RzYS5weQ==) | `72.09% <ø> (ø)` | | | [src/coincurve/flags.py](https://app.codecov.io/gh/ofek/coincurve/pull/153?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ofek+Lev#diff-c3JjL2NvaW5jdXJ2ZS9mbGFncy5weQ==) | `100.00% <ø> (ø)` | | | [src/coincurve/keys.py](https://app.codecov.io/gh/ofek/coincurve/pull/153?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ofek+Lev#diff-c3JjL2NvaW5jdXJ2ZS9rZXlzLnB5) | `76.76% <ø> (ø)` | | | [src/coincurve/types.py](https://app.codecov.io/gh/ofek/coincurve/pull/153?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ofek+Lev#diff-c3JjL2NvaW5jdXJ2ZS90eXBlcy5weQ==) | `75.00% <ø> (ø)` | | | [src/coincurve/utils.py](https://app.codecov.io/gh/ofek/coincurve/pull/153?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ofek+Lev#diff-c3JjL2NvaW5jdXJ2ZS91dGlscy5weQ==) | `87.93% <ø> (ø)` | |

[MementoRC]

Very cool. I'll go rebase the other ones.

[ofek]

Thank you!