fmt: Sort emitted entries ignoring their case

@jerome says at kirr/nxd-bom!1 (comment 185299) :

    I feel this would be more natural sorting, in the current output we have
    all the upper case first and then all the lower case.

nxdbom/__init__.py

@@ -432,7 +432,7 @@ def fmt_bom(bom): # -> str
     for kind in sorted(kinds):
         if kind != '':
             emit('\n>>> %ss:' % kind)
-        for bkey in sorted(bom):
+        for bkey in sorted(bom, key=lambda _: _[0].lower()):  # by name, ignoring case
             info = bom[bkey]
             if info.kind == kind:
                 # TODO autoalign

nxdbom/nxdbom_test.py

@@ -184,8 +184,8 @@ eggs = lxml
 -- /ROOT/eggs/Mako-1.1.4-py2.7.egg/x --
 """, """
 >>> eggs:
-Mako                         1.1.4      https://pypi.org/project/Mako/1.1.4/
 lxml                         4.9.1      https://pypi.org/project/lxml/4.9.1/
+Mako                         1.1.4      https://pypi.org/project/Mako/1.1.4/
 pycurl                       7.43.0     https://pypi.org/project/pycurl/7.43.0/
 """)
 
@@ -335,10 +335,10 @@ url = http://www.python.org/ftp/python/2.7.18/Python-2.7.18.tar.xz
 recipe = slapos.recipe.cmmi
 url = https://www.python.org/ftp/python/3.9.15/Python-3.9.15.tar.xz
 """, """\
-Python                       2.7.18     http://www.python.org/ftp/python/2.7.18/Python-2.7.18.tar.xz
-Python3                      3.9.15     https://www.python.org/ftp/python/3.9.15/Python-3.9.15.tar.xz
 libpng                       1.6.37     http://download.sourceforge.net/libpng/libpng-1.6.37.tar.xz
 libpng12                     1.2.59     http://download.sourceforge.net/libpng/libpng-1.2.59.tar.xz
+Python                       2.7.18     http://www.python.org/ftp/python/2.7.18/Python-2.7.18.tar.xz
+Python3                      3.9.15     https://www.python.org/ftp/python/3.9.15/Python-3.9.15.tar.xz
 readline                     8.1        http://ftp.gnu.org/gnu/readline/readline-8.1.tar.gz
 readline5                    5.2        http://ftp.gnu.org/gnu/readline/readline-5.2.tar.gz
 """)