Fix build - #52

Traceback (most recent call last):
  File "python-langserver.py", line 8, in <module>
    from langserver.langserver import main  # noqa: E402
  File "/src/langserver/langserver.py", line 14, in <module>
    from .workspace import Workspace
  File "/src/langserver/workspace.py", line 4, in <module>
    from .fetch import fetch_dependency
  File "/src/langserver/fetch.py", line 8, in <module>
    import pip.status_codes
ModuleNotFoundError: No module named 'pip.status_codes'