AppVeyor Linux-based CI and MacOS CI works well, like the free Windows-based CI AppVeyor is well known for. GitHub Actions CI is an even more popular choice.

Minimal working examples of .appveyor.yml for both Windows and Linux using Python, C++ and/or Fortran are given below. Use per-line distinct commands for each OS where needed. The other way is to use matrix:, but unique per-line OS commands are more intuitive for many use cases.

• For Linux, the stack: stanza picks software/version for a set of common programs–here, Python 3.
• Note the use of cmd: for Windows-exclusive commands, and sh: for Linux-exclusive commands.

Note: f2py on AppVeyor Windows has its own page–importing Fortran libraries from Python.

This appveyor.yml runs Python on the latest MacOS, Windows and Ubuntu:

image:
- macos
- Visual Studio
- Ubuntu

stack: python 3

environment:
  PY_DIR: C:\Python39-x64

clone_depth: 25

build: off

init:
- cmd: set PATH=%PY_DIR%;%PY_DIR%\Scripts;%PATH%

install:
- python --version
- python -m pip install -e .[tests,full]

test_script: pytest

Here is a Fortran example:

image:
- macos
- Visual Studio
- ubuntu

environment:
  MSYS_DIR: "c:/msys64/mingw64/bin"
  CMAKE_GENERATOR: Ninja

clone_depth: 25

build: off

init:
- cmd: set PATH=%MSYS_DIR%;%PATH%

install:
- cmd: cmake -B build
- sh: cmake -B build

- cmake --install build

test_script:
- ctest --test-dir build -V

Install additional Linux programs in AppVeyor via commands like:

init:
- sh: sudo apt -yq update > /dev/null
- sh: sudo apt install -yq --no-install-suggests --no-install-recommends bwbasic > /dev/null

• image: selects between build worker images

Command “python setup.py egg_info” failed with error code 1

Try using Miniconda3 C:\Miniconda3-x64

Related: f2py on AppVeyor Windows