Failed to install latest version of mayavi with the self-compiled git master version of vtk.

Hi,

My environment is: Ubuntu 20.04 + python 3.8.3 (managed by pyenv) + vtk git master version with python wrapper.

I try to install the latest version of mayavi use the following steps:

```
# Create a virtualenv for mayavi:
$ pyenv shell 3.8.3
# The vtk wrapper is installed in the base python enironment, hence use the following option:
$ pyenv virtualenv --system-site-packages mayavi
$ pyenv shell --unset
$ pyenv shell mayavi
$ git clone https://github.com/enthought/mayavi.git mayavi.git
$ cd mayavi.git
$ pip install -U pip
$ grep -v '^vtk' requirements.txt | xargs -n1 pip install
$ pip install PyQt5
$ pip install -e .
Looking in indexes: https://pypi.tuna.tsinghua.edu.cn/simple
Obtaining file:///home/werner/Public/hpc/tools/enthought/mayavi.git
Requirement already satisfied: apptools in /home/werner/.pyenv/versions/3.8.3/envs/mayavi/lib/python3.8/site-packages (from mayavi==4.7.2.dev0) (4.5.0)
Requirement already satisfied: envisage in /home/werner/.pyenv/versions/3.8.3/envs/mayavi/lib/python3.8/site-packages (from mayavi==4.7.2.dev0) (4.9.2)
Requirement already satisfied: numpy in /home/werner/.pyenv/versions/3.8.3/envs/mayavi/lib/python3.8/site-packages (from mayavi==4.7.2.dev0) (1.18.5)
Requirement already satisfied: pyface>=6.1.1 in /home/werner/.pyenv/versions/3.8.3/envs/mayavi/lib/python3.8/site-packages (from mayavi==4.7.2.dev0) (7.0.0)
Requirement already satisfied: pygments in /home/werner/.pyenv/versions/3.8.3/envs/mayavi/lib/python3.8/site-packages (from mayavi==4.7.2.dev0) (2.6.1)
Requirement already satisfied: traits>=4.6.0 in /home/werner/.pyenv/versions/3.8.3/envs/mayavi/lib/python3.8/site-packages (from mayavi==4.7.2.dev0) (6.1.0)
Requirement already satisfied: traitsui>=6.0.0 in /home/werner/.pyenv/versions/3.8.3/envs/mayavi/lib/python3.8/site-packages (from mayavi==4.7.2.dev0) (7.0.0)
ERROR: Could not find a version that satisfies the requirement vtk (from mayavi==4.7.2.dev0) (from versions: none)
ERROR: No matching distribution found for vtk (from mayavi==4.7.2.dev0)
```
Any hints for this problem?

Regards,
HY