Add isort third party detection from requirements.txt

The current setup used in the OCA for isort is done by using 2
pre-commit hooks:

1. seed-isort-config: find third-party libs and updates the
'.isort.cfg' file's "known_third_party" with the list
2. isort: runs isort, which will use the list of third party
libs provided by 'seed-isort-config'

This is an issue [0] with the workflow adopted by many contributors of
the OCA: as the pull requests may need some time to be merged, we use
temporary branches where we aggregate the various PRs.

If several pull requests add a third party library, we'll have a
conflict, and when the process of merging these pull requests is
automated (e.g. with git-aggregator [1]), it becomes tedious.

The list of libs in the "known_third_party" variable is not even
editable manually, as "seed-isort-config" overwrite the value on every
commit: it doesn't really make sense to actually store this.

As pointed out by @sbidoul [2], isort mentions in their changelog:

> Support for using requirements files to auto determine third-paty
> section if pipreqs & requirementslib are installed.

Which is the goal of this change, whose details are:

* Remove 'seed-isort-config' from pre-commit as the list of libs will be
  provided by requirements.txt
* Replace the isort pre-commit repo by the official repo (the mirror says
  it is been deprecated in favor of the official repo)
* Adds pipreqs and pip-api in additional dependencies to activate
  isort's feature to read requirements.txt [3]
* Adds types: [python] in the pre-commit config: the mirror had it and
  the official repo doesn't. Without it, some files such as .pot are
  modified (different EOF).
* Add an union merge driver on 'requirements.txt' to resolve conflicts
on this file (on conflicts, both lines are kept, which works in most
cases on this file)

Alternatively, we could evaluate 'reorder-python-imports' in place of 'isort' [4]

I propose to evaluate this configuration on this repository, since this
is where I ran into the issue. If the result is good (my first test
shows it should), then we can apply the same commit on https://github.com/OCA/maintainer-quality-tools

[0] OCA/maintainer-quality-tools#625
[1] https://github.com/acsone/git-aggregator
[2] OCA/maintainer-quality-tools#625 (comment)
[3] https://github.com/timothycrosley/isort/blob/500bafabbd51a6005c11a00c4738a2438990e48a/pyproject.toml#L42
[4] https://github.com/asottile/reorder_python_imports

.gitattributes

@@ -0,0 +1 @@
+requirements.txt merge=union

.isort.cfg

@@ -9,4 +9,3 @@ line_length=88
 known_odoo=odoo
 known_odoo_addons=odoo.addons
 sections=FUTURE,STDLIB,THIRDPARTY,ODOO,ODOO_ADDONS,FIRSTPARTY,LOCALFOLDER
-known_third_party=setuptools

.pre-commit-config.yaml

@@ -51,16 +51,14 @@ repos:
   rev: v1.24.0
   hooks:
   - id: pyupgrade
-- repo: https://github.com/asottile/seed-isort-config
-  rev: v1.9.3
-  hooks:
-  - id: seed-isort-config
-- repo: https://github.com/pre-commit/mirrors-isort
-  rev: v4.3.21
+- repo: https://github.com/timothycrosley/isort
+  rev: 4.3.21-2
   hooks:
   - id: isort
     name: isort except __init__.py
+    'types': [python]
     exclude: /__init__\.py$
+    additional_dependencies: [pipreqs, pip-api]
 - repo: https://github.com/pre-commit/mirrors-eslint
   rev: v6.5.1
   hooks: