Eliminate warning on Python 3.8 or later

[mkindahl]

If is not or is is used to compare literals, Python will complain on 3.8 or later. Change this to use != and == instead.

[mkindahl]

It shows up this way on my machine:

mats@fury:~$ sudo apt install gitsome
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following NEW packages will be installed:
  gitsome
0 upgraded, 1 newly installed, 0 to remove and 18 not upgraded.
Need to get 105 kB of archives.
After this operation, 691 kB of additional disk space will be used.
Get:1 http://se.archive.ubuntu.com/ubuntu jammy/universe amd64 gitsome all 0.8.0+ds-6ubuntu1 [105 kB]
Fetched 105 kB in 0s (814 kB/s)   
Selecting previously unselected package gitsome.
(Reading database ... 670661 files and directories currently installed.)
Preparing to unpack .../gitsome_0.8.0+ds-6ubuntu1_all.deb ...
Unpacking gitsome (0.8.0+ds-6ubuntu1) ...
Setting up gitsome (0.8.0+ds-6ubuntu1) ...
/usr/lib/python3/dist-packages/gitsome/github.py:792: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  ('/' + language if language is not 'overall' else '') +
/usr/lib/python3/dist-packages/gitsome/lib/html2text/html2text.py:95: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if c is not ' ' and c is not '  ':
/usr/lib/python3/dist-packages/gitsome/lib/html2text/html2text.py:95: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if c is not ' ' and c is not '  ':
/usr/lib/python3/dist-packages/gitsome/lib/html2text/html2text.py:96: SyntaxWarning: "is" with a literal. Did you mean "=="?
  return c is ' '

[mkindahl]

Closing as duplicate of #196 and #182