Build: pin pip as workaround (#8865)
* Build: pin pip as workaround
Latest pip version is failing wit
```
ERROR: Exception:
Traceback (most recent call last):
File "/home/docs/checkouts/readthedocs.org/user_builds/qtile/envs/3274/lib/python3.7/site-packages/pip/_internal/cli/base_command.py", line 165, in exc_logging_wrapper
status = run_func(*args)
File "/home/docs/checkouts/readthedocs.org/user_builds/qtile/envs/3274/lib/python3.7/site-packages/pip/_internal/cli/req_command.py", line 205, in wrapper
return func(self, options, args)
File "/home/docs/checkouts/readthedocs.org/user_builds/qtile/envs/3274/lib/python3.7/site-packages/pip/_internal/commands/install.py", line 389, in run
to_install = resolver.get_installation_order(requirement_set)
File "/home/docs/checkouts/readthedocs.org/user_builds/qtile/envs/3274/lib/python3.7/site-packages/pip/_internal/resolution/resolvelib/resolver.py", line 190, in get_installation_order
expected_node_count=len(self._result.mapping) + 1,
File "/home/docs/checkouts/readthedocs.org/user_builds/qtile/envs/3274/lib/python3.7/site-packages/pip/_internal/resolution/resolvelib/resolver.py", line 276, in get_topological_weights
assert len(weights) == expected_node_count
AssertionError
```
By pining it to the previous version we can make builds to workaround this issue
and keep building docs successfully.
See https://github.com/readthedocs/readthedocs.org/issues/8864
* Change pip selector to `<=` to support py27
Co-authored-by: Anthony <aj@ohess.org>
