Revert "percolator support gzipped file"

This reverts commit 53f7d94.

Author: RalfG

psm_utils/io/percolator.py

@@ -16,7 +16,6 @@
 from __future__ import annotations
 
 import csv
-import gzip
 import logging
 import re
 from pathlib import Path
@@ -119,12 +118,8 @@ def __iter__(self) -> Iterable[PSM]:
 
     @staticmethod
     def _read_header(filename):
-        if str(filename).endswith(".gz"):
-            with gzip.open(filename, "rt") as f:
-                fieldnames = f.readline().strip().lower().split("\t")
-        else:
-            with open(filename, "rt") as f:
-                fieldnames = f.readline().strip().lower().split("\t")
+        with open(filename, "rt") as f:
+            fieldnames = f.readline().strip().lower().split("\t")
         return fieldnames
 
     @staticmethod
@@ -378,12 +373,7 @@ def _parse_existing_file(
     ) -> Tuple[List[str], Optional[int]]:
         """Parse existing Percolator Tab file to determine fieldnames and last ScanNr."""
         # Get fieldnames
-        if str(filename).endswith(".gz"):
-            open_func = gzip.open
-        else:
-            open_func = open
-            
-        with open_func(filename, "rt") as open_file:
+        with open(filename, "rt") as open_file:
             for line in open_file:
                 fieldnames = line.strip().split("\t")
                 break
@@ -398,7 +388,7 @@ def _parse_existing_file(
 
         # Get last ScanNr
         last_scannr = None
-        with open_func(filename, "rt") as open_file:
+        with open(filename, "rt") as open_file:
             # Read last line
             open_file.seek(0)
             last_line = None
@@ -425,11 +415,7 @@ def _parse_existing_file(
 class _PercolatorTabIO:
     def __init__(self, *args, protein_separator="|||", **kwargs) -> None:
         """File reader and writer for Percolator Tab files with fixed Proteins tab."""
-        filename = args[0]
-        if str(filename).endswith(".gz"):
-            self._open_file = gzip.open(*args, **kwargs)
-        else:
-            self._open_file = open(*args, **kwargs)
+        self._open_file = open(*args, **kwargs)
         self.protein_separator = protein_separator
 
     def __enter__(self, *args, **kwargs) -> _PercolatorTabIO: