Fix byte mode detection on BgzfWriter

In commit c4a47ffff7f3e7de32ab3d8846983d3531ea63b4, there was an
attempt to detect handles which are not in binary mode. However, there's
a core problem here in that most handles open in write mode *cannot* be
read from. For example:

```python
>>> handle = open('/tmp/demo.txt', 'wb')
>>> handle.read(0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
io.UnsupportedOperation: read
```

This means that one cannot pass a plain writeable file object to
`BgzfWriter`:

```python
>>> Bio.__version__
'1.83'
>>> Bio.bgzf
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'Bio' has no attribute 'bgzf'
>>> from Bio import bgzf
>>> with open('/tmp/demo.txt', 'wb') as handle:
...     bgzf.BgzfWriter(fileobj=handle)
...
    if fileobj.read(0) != b"":
       ^^^^^^^^^^^^^^^
io.UnsupportedOperation: read
```

If eager detection of the handle's mode is desired, one can check
`handle.mode` as was done before -- I opt here to just hardcode handling
of `io.StringIO`. The simplest option in my opinion is to just rely on
ducktyping -- the `_write_block()` method will try to write the data --
either the handle accepts `bytes` and all is well, or it fails.

Bio/bgzf.py

@@ -247,6 +247,7 @@ If your data is in UTF-8 or any other incompatible encoding, you must use
 binary mode, and decode the appropriate fragments yourself.
 """
 
+import io
 import struct
 import sys
 import zlib
@@ -799,9 +800,13 @@ class BgzfWriter:
         """Initialize the class."""
         if filename and fileobj:
             raise ValueError("Supply either filename or fileobj, not both")
-        # If an open file was passed, make sure it was opened in binary mode.
         if fileobj:
-            if fileobj.read(0) != b"":
+            # If an open file was passed, make sure it was opened in binary mode.
+            # This is a courtesy -- we can't detect mode for all file-like objects.
+            # Notably, `StringIO` does not have a `mode` attribute but plain files *do*.
+            if isinstance(fileobj, io.StringIO) or "b" not in getattr(
+                fileobj, "mode", "wb"
+            ):
                 raise ValueError("fileobj not opened in binary mode")
             handle = fileobj
         else:

Tests/test_bgzf.py

@@ -8,12 +8,12 @@
 See also the doctests in bgzf.py which are called via run_tests.py
 """
 
-import unittest
 import gzip
+import io
 import os
 import tempfile
+import unittest
 from random import shuffle
-import io
 
 from Bio import bgzf
 
@@ -504,6 +504,18 @@ class BgzfTests(unittest.TestCase):
         with self.assertRaisesRegex(ValueError, error):
             bgzf.BgzfWriter(fileobj=io.StringIO())
 
+    def test_writer_with_non_binary_file(self):
+        """A BgzfWriter must raise ValueError on a non-binary file handle."""
+        error = "^fileobj not opened in binary mode$"
+        with open(self.temp_file, "w") as handle:
+            with self.assertRaisesRegex(ValueError, error):
+                bgzf.BgzfWriter(fileobj=handle)
+
+    def test_writer_passes_on_plain_file_handle(self):
+        """A BgzfWriter must be able to work with plain file handles."""
+        with open(self.temp_file, "wb") as handle:
+            bgzf.BgzfWriter(fileobj=handle)
+
 
 if __name__ == "__main__":
     runner = unittest.TextTestRunner(verbosity=2)