write_pyi.py script using globally installed dependencies

[agriffin-grow]

Describe the bug
There are two primary issues

1. The order of imports in write_pyi requires alembic installed globally for some of the imports
2. Running write_pyi also requires sqlalchemy, black, and zimports installed globally

Expected behavior
As a development script, this should be runnable from within the codebase without hard dependencies on the external system.

To Reproduce
Uninstall global alembic and sqlalchemy packages and run python tools/write_pyi.py

Error
without global alembic

$ python tools/write_pyi.py
Traceback (most recent call last):
  File "/Users/x/code/alembic/tools/write_pyi.py", line 15, in <module>
    from alembic.autogenerate.api import AutogenContext
ModuleNotFoundError: No module named 'alembic'

with alembic installed globally you get

$ python tools/write_pyi.py
Traceback (most recent call last):
  File "/Users/x/code/alembic/tools/write_pyi.py", line 16, in <module>
    from alembic.autogenerate.api import AutogenContext
  File "/Users/x/code/alembic/alembic/__init__.py", line 1, in <module>
    from . import context
  File "/Users/x/code/alembic/alembic/context.py", line 1, in <module>
    from .runtime.environment import EnvironmentContext
  File "/Users/x/code/alembic/alembic/runtime/environment.py", line 23, in <module>
    from .migration import _ProxyTransaction
  File "/Users/x/code/alembic/alembic/runtime/migration.py", line 35, in <module>
    from .. import ddl
  File "/Users/x/code/alembic/alembic/ddl/__init__.py", line 1, in <module>
    from . import mssql
  File "/Users/x/code/alembic/alembic/ddl/mssql.py", line 20, in <module>
    from .base import AddColumn
  File "/Users/x/code/alembic/alembic/ddl/base.py", line 19, in <module>
    from ..util.sqla_compat import _columns_for_constraint  # noqa
  File "/Users/x/code/alembic/alembic/util/__init__.py", line 24, in <module>
    from .pyfiles import coerce_resource_to_filename as coerce_resource_to_filename
  File "/Users/x/code/alembic/alembic/util/pyfiles.py", line 15, in <module>
    from mako import exceptions
ModuleNotFoundError: No module named 'mako'

Additional context

Fix incoming

Have a nice day!

[CaselIT]

Hi,

it's an internal tool, so it's not really that polished. I'll take a look at the PR though

[sqla-tester]

Aaron Griffin has proposed a fix for this issue in the main branch:

Improve write_pyi usage https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/5586