Enable Pylint in CI and fix its errors (riscv#311)

* Remove wildcard imports

Use explicit imports rather than wildcards. This is more maintainable.

* Enable Pylint in CI and fix its errors

The main fixes were:

* Specify encoding for all file opens. By default it depends on environment variables which is bad.
* Use `with` to open files. Otherwise they don't necessarily get closed.

There were also a few minor things like using `enumerate`, not using objects as default arguments, etc. In some cases I slightly refactored the code.

test.py

@@ -4,7 +4,25 @@
 import unittest
 from unittest.mock import Mock, patch
 
-from shared_utils import *
+from shared_utils import (
+    InstrDict,
+    check_arg_lut,
+    check_overlapping_bits,
+    extract_isa_type,
+    find_extension_file,
+    handle_arg_lut_mapping,
+    initialize_encoding,
+    is_rv_variant,
+    overlaps,
+    pad_to_equal_length,
+    parse_instruction_line,
+    process_enc_line,
+    process_fixed_ranges,
+    process_standard_instructions,
+    same_base_isa,
+    update_encoding_for_fixed_range,
+    validate_bit_range,
+)
 
 
 class EncodingUtilsTest(unittest.TestCase):