Pyro4

- fixed `expose` issue on static method/classmethod due to API change in Python 3.10
- switched from travis to using github actions for CI builds and tests
- Python 3.10 is now included in the unit test runs

- fix some typo's in docs
- corrected some minor other things in docs
- Python 3.9 is now included in the unit test runs
- No actual code changes.

- fix possible race condition when creating instances with instancemode "single"

- cython compatibility fix
- removed explicit version checks of dependencies such as serpent. This fixes crash error when dealing with prerelease versions that didn't match the pattern.

- stick to an older serpent library version to install as a dependency when installing on old python versions
- explain the delay and threading in the distributed-computing2 example

- dropped support for Python 3.4 (which has reached end-of-life status). Supported Python versions are now 2.7, and 3.5 or newer. (the life cycle status of the Python versions can be seen here https://devguide.python.org/#status-of-python-branches)
- URIs now allow spaces in the location part. Useful for unix domain sockets.