fa2 library changed to fa2_modified

[AminAlam]

This PR contains commits changing fa2 to fa2_modified as the original fa2 is not maintained anymore and doesn't work with Python3.9+ . fa2_modified has the exact same functionality as the original fa2, but its Cython codes have been modified to make it work with newer version of Python.

• [x] Closes #2067
• [x] Tests included or not required because: No additional rests are required as the already existing tests cover these changes
• [ ] Release notes not necessary because:

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.33333% with 30 lines in your changes missing coverage. Please review.

Project coverage is 76.73%. Comparing base (d4e1fb4) to head (403dd30). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scanpy/plotting/_utils.py	25.00%	15 Missing :warning:
src/scanpy/tools/_draw_graph.py	31.57%	13 Missing :warning:
src/scanpy/plotting/_tools/paga.py	60.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3220 +/- ## ========================================== + Coverage 74.17% 76.73% +2.55% ========================================== Files 109 109 Lines 12497 12529 +32 ========================================== + Hits 9270 9614 +344 + Misses 3227 2915 -312 ``` | [Files with missing lines](https://app.codecov.io/gh/scverse/scanpy/pull/3220?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse) | Coverage Δ | | |---|---|---| | [src/scanpy/plotting/\_tools/scatterplots.py](https://app.codecov.io/gh/scverse/scanpy/pull/3220?src=pr&el=tree&filepath=src%2Fscanpy%2Fplotting%2F_tools%2Fscatterplots.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NjYW5weS9wbG90dGluZy9fdG9vbHMvc2NhdHRlcnBsb3RzLnB5) | `87.02% <100.00%> (+3.61%)` | :arrow_up: | | [src/scanpy/plotting/\_tools/paga.py](https://app.codecov.io/gh/scverse/scanpy/pull/3220?src=pr&el=tree&filepath=src%2Fscanpy%2Fplotting%2F_tools%2Fpaga.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NjYW5weS9wbG90dGluZy9fdG9vbHMvcGFnYS5weQ==) | `68.47% <60.00%> (+1.48%)` | :arrow_up: | | [src/scanpy/tools/\_draw\_graph.py](https://app.codecov.io/gh/scverse/scanpy/pull/3220?src=pr&el=tree&filepath=src%2Fscanpy%2Ftools%2F_draw_graph.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NjYW5weS90b29scy9fZHJhd19ncmFwaC5weQ==) | `65.57% <31.57%> (+3.07%)` | :arrow_up: | | [src/scanpy/plotting/\_utils.py](https://app.codecov.io/gh/scverse/scanpy/pull/3220?src=pr&el=tree&filepath=src%2Fscanpy%2Fplotting%2F_utils.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NjYW5weS9wbG90dGluZy9fdXRpbHMucHk=) | `56.23% <25.00%> (+0.53%)` | :arrow_up: | ... and [28 files with indirect coverage changes](https://app.codecov.io/gh/scverse/scanpy/pull/3220/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse)

[Zethson]

Could you please add a release note?

[AminAlam]

Could you please add a release note?

Sure; I added it.

[flying-sheep]

Thanks!