Remove Exhale from requirements and regenerate

Regenerate the requirements.txt with Exhale removed and also with Python
3.9 instead of 3.8, for parity with Read The Docs.

Signed-off-by: David Horstmann <david.horstmann@arm.com>

docs/requirements.in

@@ -1,3 +1,2 @@
 sphinx-rtd-theme
 breathe
-exhale

docs/requirements.txt

@@ -1,5 +1,5 @@
 #
-# This file is autogenerated by pip-compile with Python 3.8
+# This file is autogenerated by pip-compile with Python 3.9
 # by the following command:
 #
 #    pip-compile requirements.in
@@ -8,12 +8,8 @@ alabaster==0.7.13
     # via sphinx
 babel==2.12.1
     # via sphinx
-beautifulsoup4==4.11.2
-    # via exhale
 breathe==4.35.0
-    # via
-    #   -r requirements.in
-    #   exhale
+    # via -r requirements.in
 certifi==2022.12.7
     # via requests
 charset-normalizer==3.1.0
@@ -21,11 +17,8 @@ charset-normalizer==3.1.0
 docutils==0.17.1
     # via
     #   breathe
-    #   exhale
     #   sphinx
     #   sphinx-rtd-theme
-exhale==0.3.6
-    # via -r requirements.in
 idna==3.4
     # via requests
 imagesize==1.4.1
@@ -34,28 +27,19 @@ importlib-metadata==6.0.0
     # via sphinx
 jinja2==3.1.2
     # via sphinx
-lxml==4.9.2
-    # via exhale
 markupsafe==2.1.2
     # via jinja2
 packaging==23.0
     # via sphinx
 pygments==2.14.0
     # via sphinx
-pytz==2022.7.1
-    # via babel
 requests==2.28.2
     # via sphinx
-six==1.16.0
-    # via exhale
 snowballstemmer==2.2.0
     # via sphinx
-soupsieve==2.4
-    # via beautifulsoup4
 sphinx==4.5.0
     # via
     #   breathe
-    #   exhale
     #   sphinx-rtd-theme
 sphinx-rtd-theme==1.2.0
     # via -r requirements.in