Report Flake8 errors with Error severity level

Error codes from Flake8 :
  - Fxxx : checks from PyFlakes => Error severity level
  - E999 : code for invalid AST => Error severity level
  - Exxx : checks from pydocstyle (syntax checks for PEP-8) => Warning
    severity level
  - Wxxx : checks from pydocstyle (syntax checks for PEP-8) => Warning
    severity level

Closes python-lsp#230
Fix PR python-lsp#223

pylsp/plugins/flake8_lint.py

@@ -174,7 +174,7 @@ def parse_stdout(document, stdout):
         # show also the code in message
         msg = code + ' ' + msg
         severity = lsp.DiagnosticSeverity.Warning
-        if code[0] == "E":
+        if code == "E999" or code[0] == "F":
             severity = lsp.DiagnosticSeverity.Error
         diagnostics.append(
             {