Fix byte mode detection on BgzfWriter

[DavidCain]

In commit c4a47ff, 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:

>>> 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:

>>> 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.

[peterjc]

Oh my, we missed that on #4270 - and therefore we're missing something on the test suite too.

[peterjc]

Do you agree to dual licence this and any previous contributions under both the Biopython License Agreement AND the BSD 3-Clause License.

Also do you want to be named in the NEWS file etc?

[DavidCain]

Do you agree to dual licence this and any previous contributions under both the Biopython License Agreement AND the BSD 3-Clause License.

Also do you want to be named in the NEWS file etc?

Yes, I agree to dual-license this work, and sure -- feel free to mention me in NEWS and such (I should already be in CONTRIB.rst!). Thanks, @peterjc

[peterjc]

I'll merge it in the next few days (leaving you a chance to update the NEWS file to add your name if you want). Thanks!

[DavidCain]

Thanks, @peterjc -- just added!